Lore: Malachai & Friends
Malachai, Thaumaturge Laureate
(If you're not interested in the storyline, you can safely skip this section.)
By this point, you've probably noticed a recurring name in the lore items on the ground: Malachai.
Malachai was an expert in thaumaturgy who lived in the last days of the Eternal Empire. He was the Thaumaturge Laureate of the Empire under Emperor Chitus, the second to last, and advanced the science of thaumaturgy to new heights along with his students Maligaro, Doedre, and Shavronne.
When the Purity Rebellion overthrew Emperor Chitus, things ought to have ended. High Templar and soon to be Emperor Voll wanted thaumaturgy gone from the Empire, and by all rights Malachai should have met his death then. But Malachai persuaded the new Emperor that he could be of use - that he could exterminate thaumaturgy once and for all by destroying its source: a creature located under Mount Veruso known only as "The Beast".
This, obviously, did not happen. Instead, the Cataclysm obliterated the Eternal Empire, leaving only a few scattered survivors. Yet the power of the Beast remained.
The Depraved Trinity
Malachai's greatest students have been in the background so far, but are worth discussing a little more in their own right.
Shavronne of Umbra was a noblewoman obsessed with beauty. She detested the world and everything in it: too ugly, too imperfect. Instead, she turned to thaumaturgy, which she saw as holding the power to fix a twisted world by twisting it further. During the Purity Rebellion, she attempted to stop the invasion of the Karui (led by King Kaom) along the coast, and eventually turned to transforming Brutus into a hulking monster as a last-ditch attempt to stop the Karui advance. She was killed by her creation not long afterward.
Maligaro, like Shavronne, viewed thaumaturgy as an artistic endeavor - but unlike Shavronne's disgust, Maligaro had no particular emotional stake in his work. He was neutral, in the most terrible sense, and wanted only to learn to craft ever more artistic forms in flesh. The Chamber of Sins, in Act 2, is Maligaro's work - as is the beast known as Fidelitas, a mutated form of Maligaro's former lover. High Templar Voll had Maligaro burned at the stake after the successful Purity Rebellion.
Doedre Darktongue has, up until now, been mentioned less: she's only been encountered as a proclamation for her execution in the Western Forest. Like Maligaro, she was executed by Voll after the Purity Rebellion.
Lady Dialla, the Gemling Queen
Malachai's consort Dialla is the only apparent survivor of the Cataclysm, who you met back in the Solaris Temple.
She was apparently a concubine of some sort to Emperor Chitus at one point but, as she puts it, 'asked too many questions', and was gifted to Malachai. Malachai, for his part, was happy to sate her curiosity for Nightmare, the power of the Beast.
As best as can be pieced together from dialog, Dialla did genuinely love Malachai, but his feelings for her are less clear. At a minimum, Malachai viewed his work on her body as his greatest accomplishment, hence 'the Gemling Queen'.
The Beast is a creature of some sort that "lives" (insofar as something of its power can be said to "live") in the bowels of Mount Veruso, where you're headed in Act 4. Its power, known as "Nightmare", appears to be the source of thaumaturgy.
Grigor, an Act 3 NPC, describes the Beast as follows:
When Piety was... experimenting on me, my consciousness was mercifully fleeting. In those moments of numbing darkness, I met a presence. Intelligence, power, immensity beyond the limits of my pitiable moral senses. To this creature, I was but a rain drip falling into the sea.
I heard Piety speak to her lackeys of 'The Beast'. It is the source of her thaumaturgy, and the object of her ambitions. I believe Piety's 'Beast' and the dark entity are are one and the same. Wherever it is, whatever it is, The Beast is the cause of my malformation. It wouldn't be a stretch of reason to consider The Beast the source of all malformation in Wraeclast.
It was the Beast that Malachai promised Voll that he would kill - and, apparently, what he failed to kill when prompted to do so. The Beast remains, sealed inside the mountain. The thing about Nightmares, though, is that they wake up...