Jun & the Syndicate
The Month of Jun
As you enter Act 9, you'll meet Jun Ortoi in town. She'll ask for your help fighting the Syndicate, a hostile organization of mysterious origins. She's the fourth main Master you'll meet throughout the campaign and, like Einhar, Niko, and Alva before her, she introduces a major side progression mechanic to the game.
The Immortal Syndicate
A handful of previously-friendly NPCs have joined forces with an array of characters drawn from Path of Exile's lore to form the Immortal Syndicate. The members of this syndicate do not stay dead - hence the name - and so you will fight them repeatedly over time.
Your goal is to gain intelligence on the Syndicate, raid their Safehouses for loot, and ultimately face their Mastermind in a major endgame boss fight. On your way, you'll manipulate relationships among the Syndicate members (ever play Shadow of Mordor?) to raise or lower their ranks and shuffle them around within the Syndicate's different wings. Doing so lets you modify the difficulty - and loot - of the safehouse raids and ultimately of the Mastermind encounter itself.
Much like Einhar, Niko, and Alva, Jun will appear periodically in random zones. When she does, that zone will include several Syndicate encounters (typically three) of various types.
Betrayal's mechanics are complex, but fairly rewarding. You can, if you so choose, skip this section and return to it later when you're ready for more complexity. Continue on for the details.
The Investigation Board
Once you've interacted with Jun Ortoi, you can speak to her to bring up the Investigation Board. It displays the relationships among the various Syndicate members, along with their ranks and positions within the organization, as well as your progress towards raiding each Safehouse.
The Mastermind is located at the top of the screen - in this image, they're represented by a red question mark. Below the Mastermind are the four main departments of the Syndicate: from left to right, Transportation, Fortification, Research, and Intervention. Each department has its own associated missions when you encounter it in the wild, and each is led by one of the members of the Syndicate. In this image, three of the four department leaders are revealed. Each department leader's panel shows the progress towards raiding that department's Safehouse: in this case, the player is close to ready to raid Transportation, is already prepared to raid Research, and has a ways to go before raiding Fortification or Intervention.
Below the department leaders are the rank and file. Most of them are associated with a particular department - for example, Janus (far left) is associated with Transportation in this image, and Vorici (far right) is associated with Intervention. Which department a member is associated with determines the loot that they'll drop when defeated as part of a Safehouse raid (and only members associated with a particular department will appear in its Safehouse). If a department head is demoted (usually by defeating them during a Safehouse raid), one of their underlings will take over the department.
Each time you defeat a member of the Syndicate, you'll be presented with a choice of two options, which will vary depending on the circumstances in which you fought them. If you fight multiple members of the Syndicate at once, these options will change, and once you make a choice for one member the other options will change depending on your choice.
The choices available depend on the relationships between the Syndicate members and how many of them appeared. For example, "Execute" (which, because they're immortal, actually upgrades the member in question) will only appear if two or more Syndicate members were involved in the fight.
Ranks and Affiliations
Each member of the Syndicate has a rank, which can be found on their panel within the Investigation Board. They can be rank 0 (not shown), or ranks 1, 2, or 3 (displayed as a corresponding number of stars). In the image below, Riker Maloney is Rank 1, but Elreon (top left) is Rank 3.
Any member of the Syndicate above Rank 0 is associated with one of the Syndicate's departments. In this case, Riker is associated with Intervention, along with being Rank 1. A rank 0 member who becomes affiliated with a department through another member's bargaining options will become rank 1 even if not otherwise noted.
Higher-rank members of the Syndicate gain items that increase their power for each rank they gain. In this case, Riker has an item that increases his Cast Speed. These items aren't drops, they're just buffs applied to the Syndicate member, and they can be removed through one possible option after a successful fight. Higher-rank Syndicate members drop more items during a Safehouse raid.
Which department a particular Syndicate member is associated with, on the other hand, determines the type of drops they give. Some Syndicate members may have drops you're not interested in in one department - if so, try shuffling them to another one, where they'll have different loot options. You can find a nice cheat-sheet of these options here.
In addition to the Syndicate's own mechanics, Syndicate members can drop items with a special Veiled modifier, indicated by a line of weird scrambled text. You can speak to Jun Ortoi to "unveil" these modifiers, which gives you a list of three options to choose from.
Choosing one will apply the modifier in question to the item. Moreover, it will "level up" that Veiled modifier for the future - the more times you choose a particular Veiled modifier, the more powerful it becomes up to a limit.
Once you've unlocked a Veiled Modifier by Unveiling it enough times, it becomes available for crafting at the Crafting Bench in your hideout.