Source: Beyond the Veiled Past, pg(s). 73
In the Age of Darkness, Thulgroon forged an alliance with thousands of shriezyx, one based entirely on benefit, as the shriezyx cannot be magically controlled; this also ensures that Thulgroon's foes cannot glean its secret plans from the shriezyx. Akorsavatha, whom it uplifted from the shriezyx stock, serves as the intermediary between Thulgroon and the lesser shriezyx. Ouvoor, an uplifted chuul, serves as commander of Thulgroon's chuul servants, and Sicuro serves as its spy on the surface. Thulgroon also counts many underlings among Magnimar's elite.
Thulgroon's presence was discovered by the Bonds of the Chained Eyes, a cult of Kroina, whose leader Ehawi Sees-All-Words has been trying hard to uncover its true identity since then. The cult knows that Thulgroon is an alghollthu operating in the undercity, but mistakenly believes it to be a mere alghollthu master. They have found no candidate trustworthy and capable enough to defeat Thulgroon.
Thulgroon survived Earthfall by hiding itself in the Darklands, miles below Magnimar. It re-emerged centuries afterward, and began to build an information network by kidnapping local Varisians. As Magnimar was resettled, Thulgroon built an immense lair beneath the city under its control.
Due to its long isolation, Thulgroon came to believe that the alghollthus' downfall came from their staunch atheism, and that they instead need to command deific power so they can understand the cosmos and hold dominion over the gods as well. Thus it adopted a goal alien to the alghollthus: becoming a deity.
Thulgroon ruled out spontaneous ascension as unreliable and the Test of the Starstone as impractical. Through the ascension of Irori through self-perfection, Thulgroon believes that the secret to divinity comes from the Eternal Ember and the alghollthus' genesis. To that end, it has conducted numerous experiments in the Thassilonian fleshwarping laboratories beneath Magnimar, leaving behind countless living failed experiments, while keeping its research hidden from its kind lest it invites their reprisal before it can achieve divinity.