The Key To Success
The largest passives on the tree are the Keystone Passives. Most passives just give some sort of stat bonus, like % Increased Spell Damage or % Increased Attack Speed, but Keystones are special and usually have much larger and wider-ranging effects.
Keystones are an essential part of most builds. Most Keystones have drawbacks as well as benefits, and Keystones should not be taken without a good understanding of those drawbacks. This page walks through the mechanics of each Keystone and where you can find them on the passive tree. Note that most Keystones provide More/Less effects, not Increased/Reduced, and are therefore much more powerful than standard passives.
Some Keystones appear on items. For example, the Xoph's Blood amulet gives the Avatar of Fire keystone. Since you either have a Keystone or you don't, taking the Keystone in addition to using such an item has no additional effect (even numerical effects don't stack). However, some items have effects very similar to Keystones that do not explicitly bear the name of a Keystone, and those effects do stack with the corresponding Keystone.
Acrobatics & Phase Acrobatics
(Outside right of the passive tree. Phase Acrobatics is behind Acrobatics and can't be taken without taking Acrobatics first.)
Acrobatics (and its secondary passive Phase Acrobatics) give a chance to Dodge Attacks (Acrobatics) or Spells (Phase Acrobatics). Dodge is frequently confused with Evasion, but Evasion is a totally separate stat. Dodge simply gives you a chance to not be Hit by a skill at all, doesn't have any of the bad-luck protection that Evasion does, and isn't opposed by enemy Accuracy.
The drawback for this substantial chance to not be Hit is a major penalty to your Armour, Energy Shield, and Block Chance (but notably not Evasion). In practice, Acrobatics is a common pick for Evasion-based characters and is rarely taken by non-Evasion-based characters, since major penalties are usually not worth the Dodge chance to them.
(Top outside left of the passive tree.)
Ancestral Bond allows you to place one additional Totem at a time, effectively doubling your Totems' potential damage output if you have no other source of additional Totems (and substantially increasing their output even if you do). The drawback is that you cannot deal damage yourself.
The wording here is very precise. You cannot deal damage, but you can still Hit. For example, effects like Blind on Hit still work, even though your Hits are dealing no damage. Effects that depend on dealing damage, like Ailments, will fail. Moreover, you cannot deal damage yourself, but you can still deal it indirectly through Traps, Mines, and Totems. There are a number of weird edge cases here, so if in doubt you should do some research.
Ancestral Bond is effectively mandatory for totem builds, and is rarely used outside of them.
(Bottom outside right of the passive tree.)
Arrow Dancing gives you More chance to Evade Projectile Attacks and Less chance to Evade Melee Attacks. Note that this is more/less chance, not more/less rating. For example, if you had a 50% chance to Evade Attacks by default and Arrow Dancing gave you 40% More Chance to Evade Projectile Attacks, you'd now have a 70% chance to Evade Projectile Attacks (i.e., 40% more than your original 50%).
Avatar of Fire
(Top outside left of the passive tree.)
Avatar of Fire converts 50% of Cold, Lightning, and Physical damage to Fire. (Note that it does not convert Chaos damage to Fire.) This means that skills that normally don't deal Fire damage can often be converted entirely to Fire through the use of items that also add conversions, enabling Fire-centric builds using those skills.
The drawback is a big one: you can no longer deal any non-fire damage. Any non-Fire damage you deal is simply set to 0. Non-Fire damage sources still Hit, and can still apply on-Hit effects (see Ancestral Bond above for more on this) but can't inflict damage.
Normal damage conversion rules apply. For example, Damage Over Time cannot be converted. See the page on How Damage Works for more on conversion.
(Bottom outside left of the passive tree.)
Blood Magic sets your maximum Mana to 0 and causes all your skills to consume your Life as though it were your Mana. Effects that cost Mana will cost Life instead; effects that reserve Mana will reserve Life instead.
You are not considered to be on Full or Low Mana while Blood Magic is active - you simply do not have a mana pool at all.
You cannot kill yourself with Blood Magic. You won't be able to use a skill that would consume more than your current Life total.
The Blood Magic Support gem offers a similar effect. It causes specific skills to cost Life instead of Mana without removing your mana orb, but it does so at a substantial cost multiplier and takes up a support gem slot.
(Top outside right of the passive tree.)
Chaos Inoculation makes you entirely immune to Chaos damage, which is the only type of damage that normally bypasses Energy Shield. However, it also reduces your Life to 1, making your Energy Shield your only source of protection.
Increases and reductions to Life no longer affect your Life total, which is overriden by Chaos Inoculation's "set to 1" effect. However, things like Ailment durations and chance to to be Stunned still depend on your Life before Chaos Inoculation takes effect, so these bonuses do have some slight value to a player with Chaos Inoculation.
You are always On Full Life with Chaos Inoculation, since you always have 1/1 Life (unless you die, of course).
If you somehow lose Life directly with Chaos Inoculation, you immediately die. Effects that cost Life (for example, skills with Blood Magic) can't be used.
Chaos Inoculation is usually taken by Energy Shield builds in the endgame, when they've racked up enough total Energy Shield to survive on that alone.
(Up and to the right of the center of the passive tree.)
Conduit's language is a little confusing. It says you "share" charges with nearby party members, but what it means is that anytime you gain a Power, Frenzy, or Endurance Charge (see the page on Charges for more on these), nearby party members gain one as well.
Minions are not party members and do not benefit from this effect, so this effect is essentially useless if not in a party. It can be quite strong for support characters along with charge-generating effects, however.
(Bottom center-left of the passive tree.)
Crimson Dance modifies your Bleed effects (see the page on Ailments for more on Bleeds).
Normally, Bleeds deal a percentage of the inflicting Attack's Physical damage, deal extra damage when the target is moving if applied by a player, and are limited to a single copy dealing damage at a time. Crimson Dance allows up to eight bleeds, but halves their damage and removes the bonus to moving targets.
This passive is effectively required for a build relying on Bleed for sustained damage.
(Top outer center of the passive tree.)
Eldritch Battery moves your Energy Shield to your Mana globe instead of your Life globe. When you use a skill that costs Mana, you'll now consume your Energy Shield instead of your Mana. This can allow you to cast even if your entire Mana pool is reserved.
Effects that are based on the amount of Energy Shield you have continue to function as normal. For example, Righteous Fire deals damage to you based on your Life and Energy Shield and continues to do so even if your Energy Shield is protecting your Mana.
Spending Energy Shield on a skill does not count as damage to Energy Shield, and does not affect its normal recharge mechanics.
Eldritch Battery can be combined with another keystone, Mind Over Matter, which diverts damage from your Life to your Mana. If you have both Keystones, the diverted damage damages Energy Shield first, interrupting its recharge as normal.
(Left inner center of the passive tree.)
Elemental Equilibrium is very often misunderstood. The exact text of the passive is:
Enemies you hit with Elemental Damage temporarily get +25% Resistance to those Elements and -50% Resistance to other Elements.
This text is accurate but easy to misread.
"Elemental Damage" here is, as is normal for PoE's language, Cold, Fire, or Lightning damage (but not Physical/Chaos). So, for example, if you hit an enemy with pure Fire damage, that enemy gains 25% Fire Resistance but loses 50% Cold and Lightning Resistance.
The confusion comes when you deal more than one damage type. For example, suppose you deal all three. It's a common mistake to assume that this applies three separate effects, granting +25% resistance from one of them and then two copies of -50% from the other two, resulting in a net gain. In fact, all elements that were present in the hit gain 25% resistance, and all elements that were not lose 50%. Hitting with more elements in a single hit is actively bad when using Elemental Equilibrium; you want to alternate hits of pure damage types.
Any new Hit of Elemental Damage will override the previous effect entirely.
Note that Elemental Equilibrium is only triggered by damaging Hits, but the resulting gained/lost resistance can affect Damage Over Time. So, for example, you could hit with Cold damage, then use Scorching Ray (a Fire Damage Over Time effect) and Scorching Ray would benefit from the reduction in fire resist without further triggering Elemental Equilibrium.
Totems, Traps, and Mines deal damage with your skills, and therefore inherit the effects of Elemental Equilibrium. Minions, however, deal their own damage and won't trigger Elemental Equilibrium unless they somehow have the effect independently of you (but they can still benefit from the applied resistance drop on the target).
Elemental Equilibrium is used in many Elemental Damage builds along with triggered skills that deal types other than their main damage source or persistent multi-hit skills like Orb of Storms.
(Top left of passive tree.)
Elemental Overload removes the bonus damage on Critical Strikes and the Ailments they apply. In return, it gives you a large More Elemental Damage modifier for a period of time after dealing a Critical Strike.
Note that even marginal amounts of Critical Strike are enough to maintain good uptime on the Elemental Overload effect, so this is usually used in non-Critical-Strike-based builds.
(Top outer right of the passive tree.)
Ghost Reaver causes your Life Leech effects to be Energy Shield Leech instead. Effects that otherwise modify Life Leech will not apply to the converted Energy Shield Leech. It also doubles the maximum rate of Energy Shield Leech and reduces the rate at which your Energy Shield Recharges. (Remember, Recharge is a different mechanic, so these effects are independent of one another.)
(Bottom outer left of passive tree.)
Iron Grip causes the Melee Physical Damage bonus from Strength to also apply to Projectile Attack Physical Damage. There's no direct drawback except that projectile attacks are usually used by Dexterity-focused builds, so getting a large amount of Strength in one is potentially awkward. Almost mandatory for any Strength build using Projectile Attacks.
(Bottom outer right of passive tree.)
Iron Reflexes converts all your Evasion Rating into Armour and removes the innate bonus to Evasion given by Dexterity. The normal rules for converted values apply: Evasion converted to Armor is affected by modifiers to both stats.
For example suppose you have 30% Increased Evasion and 50% Increased Armor, 50 base Armour, and 100 base Evasion. The Evasion gets converted and is affected by both modifiers, and thus contributes 100 base * (1 + 0.3 [increased evasion] + 0.5 [increased armour]) = 180 Armour. The Armour is affected only by the Increased Armour modifier, and thus contributes 50 base * (1 + 0.5 [increased armour]) = 75 Armour, for a total of 255 Armour.
Mind Over Matter
(Top left of passive tree.)
Mind Over Matter diverts 30% of the damage inflicted on your Life to divert to your Mana instead. It does not divert damage done to Energy Shield. You still die if you run out of Life, even if you have some Mana remaining. Reserved Mana isn't available to take diverted damage. If you run out of unreserved Mana, your Life takes the full brunt of the damage.
One way to think about Mind Over Matter is as an effective ~43% boost to your effective Life, provided you have enough Mana to divert damage. For a single hit, you need mana equal to 43% of your Life. For more sustained damage, you'll need to be able to recover Mana at at least 43% the rate you recover Life (after accounting for spell costs) to avoid running out of mana due to incoming damage.
Mind Over Matter applies to all incoming damage, including Damage Over Time.
(Outside top of passive tree.)
Minion Instability causes your Minions to detonate when they reach Low Life (<35% of their maximum Life total), killing the Minion and dealing a percentage of its maximum Life as Fire damage to nearby enemies. This can add a lot of damage, but effectively reduces your Minions' Life totals by 35% (since they die immediately on reaching 35% or less).
The damage done by the explosion is affected by modifiers to the Minion's Life or Damage (including those from linked Support gems), but not by modifiers to your damage (as is usually the case with Minions).
Minions that reach the end of a limited duration die without ever going to Low Life and therefore do not explode. The same applies for minions that die because more than their maximum number are summoned.
(Bottom outside left of passive tree.)
Note: Mortal Conviction is gated behind Blood Magic, another Keystone, and can't normally be accessed without taking Blood Magic.
Mortal Conviction limits you to one active Aura (aside from Banner skills) skill at a time. In exchange, it removes the Mana reservation cost of that one Aura. Since you cannot normally have Mortal Conviction without having Blood Magic, this is effectively removing the Life reservation cost of that one Aura.
Mortal Conviction doesn't prevent you from benefiting from other players' auras, and doesn't affect non-Aura skills that reserve Mana.
(Just right of the center of the passive tree.)
Necromantic Aegis causes stats on your equipped shield to affect all your minions, but not you. This means you can no longer block and no longer benefit from stat bonuses on the shield, but your Minions now can block and benefit from those bonuses. These bonuses can include things like unique modifiers.
Each minion effectively has the shield equipped separately, it's not divided or diluted among the minions in any way. Minions do not inherit your passive bonuses unless specifically stated otherwise, so your bonuses to chance to block or shield defenses do nothing if you have Necromantic Aegis.
You're still considered to have the shield equipped, and socketed gems in your shield remain with you (and are not transferred to your minions).
Necromantic Aegis is usually used with minion-summoning builds along with shields that provide powerful effects to allies.
(Top right of passive tree.)
Pain Attunement gives a large More Spell Damage bonus when you are On Low Life (below 35% of your maximum Life). This is a powerful damage boost, but being On Low Life typically takes a specialized build to maintain. Pain Attunement has no direct drawbacks, and can be taken on non-Low Life builds for a bonus when you do happen to be low from incoming damage, but this is usually not done.
Typically, Pain Attunement is used with builds that rely on Energy Shield. They Reserve more than 65% of their Life using things like Blood Magic Support and have some way to avoid letting Chaos Damage penetrate the shield and hit their very small Life total.
(Right-center of the passive tree.)
Perfect Agony deals with Ailments. Before reading the rest of this segment, see that page and make sure you understand it.
Normally, Ailments have a set modifier of 150% damage for Critical Strikes, regardless of your Critical Strike Multiplier. Perfect Agony causes your normal Critical Strike Multiplier to affect your Ailments as well (with reduced effect), at the cost of a large Less Damage penalty to your Hits.
Perfect Agony is taken only in very specific critical-strike-based Ailment builds, neither of which are recommended for new players.
(Bottom outer right of passive tree.)
Point Blank makes your Projectile Attacks (not Spells) deal More damage to very nearby targets, but Less damage to distant ones.
More properly, Point Blank is based on the total travel distance of a Projectile Attack. Spectral Throw, for example, reverses direction and comes back to you, but is still the same projectile, and so it suffers from reduced damage on its way back. The same applies for Projectiles that Pierce, Fork, or Chain.
The Point Blank Support Gem applies this effect to a single skill. It does not stack with the Point Blank Keystone, even though it doesn't explicitly say it grants Point Blank.
Projectile Attacks that target an area deal damage based on an enemy's distance from the AoE's center, not distance from you.
(Outer left of the passive tree.)
Resolute Technique prevents enemies from Evading your Attacks (that is, prevents you from 'missing' with melee attacks), but also prevents you from ever dealing Critical Strikes.
Since you cannot miss, Accuracy is normally irrelevant, although effects based on your Accuracy (like the Juggernaut's % Attack Speed per X Accuracy) still apply. Since you cannot Critical Strike, you can never trigger effects that rely on Critical Strikes (like Elemental Overload).
(Top outer left of passive tree.)
Runebinder allows you to attach a second Brand skill to a single enemy. Normally you can only attach one Brand to an enemy, but Runebinder allows you to attach a second. It also reduces the maximum number of Totems you can summon by one; for most builds this means you can no longer place a Totem (although you'll still be able to if you have another source of extra maximum Totems).
This Keystone is essentially mandatory for Brand-focused builds.
(Bottom left of passive tree.)
Unwavering Stance removes your chance to Evade attacks, meaning your Evasion stat does nothing. (You still have Evasion, so you can still convert it using Iron Reflexes, described above.) In return, it makes you immune to Stuns. This is an easy pickup for most Armour-based builds.
(Bottom center of the passive tree.)
Vaal Pact doubles both the amount of Life Leech you apply and the maximum cap on Life Leech, but disables Life Regeneration completely. Helpful while in combat, but can be annoying outside of combat, where you can often find yourself without a good way of recovering lost Life. (Note that Regeneration is one specific type of Life gain, and other types - like Recovery from Flasks - still apply.)
(Top center right of passive tree.)
Wicked Ward makes damage not interrupt your Energy Shield's recharge if it started Recently (i.e. in the last 4 seconds). Damage is still subtracted from your Energy Shield pool, but its recharge continues as normal. Note that Recharge is not Regeneration, and neither is equivalent to Recovery. Recharge is the passive replenishment of your Energy Shield after a few seconds of not taking damage.
The drawback to this uninterruptable Recharge is a large penalty to your Regeneration and Energy Shield Leech. If your build isn't overly reliant on these, Wicked Ward can be quite beneficial.
(Top outer left of passive tree.)
Zealot's Oath causes your Life Regeneration to apply to Energy Shield instead. It converts both flat ("X Life Regenerated Per Second") and percentage ("X% of Maximum Life Regenerated Per Second") regeneration. Converted percentage regeneration uses your maximum Energy Shield, not your maximum Life.