Nexetia (pronounced Nex-e-tee-a) the Herald of Insanity is the only known Daemon Prince that began as a servant of a Tristitian. Once a psyker born to the low levels of a Hab-Block, she considered to be among some of the most powerful amongst those who entered the service of Tzeentch as a mortal.
She was once a low-born psyker in the filthiest darkness of a hab-block on an unnamed Hive World. Growing up in such an environment led to her holding resentment for the higher echelons of society as well as a strong desire to elevate herself above and beyond her desparate circumstances. She was never discovered by those of the Black Ships since not only were visits from these ships infrequent but also because she was well-hidden by her community.
Seeking to better herself as well as elevate her and her community, she devoted herself to the Trititians in what was later called the Undertaking of Madness, a process by which a person dedicates themselves wholly to the service of the Trititians in return for the greater gifts these entities could offer. Others followed her willingly, seeing the potential benefits that could be bestowed upon them.
She went on to start the Cult of Insanity which quickly spread across the underbelly of the city before spreading across the planet. Within a few short months, Nexetia had started a civil war on a planetary scale and - within the year - had conquered the planet. Due to the remote nature of the Sector as well as there being no elite forces such as the Adeptus Astartes looking over the Sector, she and her followers had not yet fought any single force stronger than an Astart Militarum Regiment.
The Corruption of a Sector
With the power to command what her loyalist forces as well as all the remaining military resources of the planet, she was now in a position to launch her outward conquest of the surrounding systems. By this point, she had already been granted substantial power by the Trisitians she worshiped. She also had substantial psychic control over legions of survivors who had refused to serve her. According to reports at the time that had escaped the planet, she towered over others as an Astartes might, had noticeable physical mutations beyond her size, and was incredibly powerful psychically. As a final note in almost all of these reports, there was an element of insanity that many only expected to see amongst the daemons and corrupted zealots she commanded - she was, in two words, incredibly unstable.
She went on to start her conquest of a now purged sector, with all that is known is that it was to the extreme galactic west beyond the Eye of Terror. With the conquest all but complete, she had truly become a Champion of Cahos. She then moved her sights onto a number of neighbouring sectors, launching massive incursions with her cult which had now become reminiscent of a set of disorganized Astra Militarum regiments rather than a cult.
As a reward for her further conquests and sheer number of followers, she was elevated by her patron god to a Daemon Prince which inevitably meant shedding her loyalty to the Tristitians and planting it solely in the hands of Tzeentch. The Tristians that corrupted her became incredibly wrathful, trying to re-establish their authority over Nexetia. The Tristitians which attempted to try to control a Daemon Prince saw the wrath of Tzeentch being brought to bear, their complete destruction being the inevitable conclusion.
Not long after her ascension into daemonhood, the Wrath Hammers and the Inquisition were finally able to intervene. Because of her status as a Daemon Prince of Tzeench, it was incredibly difficult to get to her let alone defeat her. With open Warp Rifts, potent Sorcerers and psychically enslaved legions of cultists at her command the final battle was bloody and extremely difficult, with many psychers from across the Imperial Forces suffering extreme psychic trauma or corruption.
She was subsequently banished to the Warp in the ensuing crusade which was very costly for the Imperium.
It is unknown whether or not she was every summoned back into the material universe, but it is widely assumed that she hasn't. There have been several recorded attempts of followers of Tzeentch attempting to summon her to the material universe, but they have all been thwarted, with most being thwarted by the Inquisition and/or their Chamber Militant factions.
It is also believed that should she ever return to the material universe, her power would cause catastrophic damage due to her incredible psykic capabilities.