The Advisor

"Your empire dominates all. Could you rival the gods themselves? I think not, fool! (Evil Laugh)"
- – The Advisor after the end of the Warriors of Chaos campaign
"OGRES MY LORD! THESE BRUTISH NOMADS ARE AS STRONG AS THEY ARE STUPID!"
- – The Advisor's far more memorable quote
The Advisor is an unnamed character from Total War: WARHAMMER, who is most notable for being its first OC (appearing in the first trailer of TWW1) and (unlike advisors in other Total War games) a major plot character in the first and third games (he also appeared in TWW2, but we'll get to that later).
Early Days
The first time we see him is in the first game's announcement trailer.There, The Advisor is an Imperial Light Wizard who is desperate in his attempts to save The Empire. So desperate, that he decided to study the forbidden book called the Tome of Fates, which, quite predictably, turned out really bad. The book allows the Advisor to gain knowledge of the past, present, and future, but he can only use the book in service to someone else and not himself. The book also makes it so anyone he tries to strike a deal with will at least hear him out first, which explains how he was able to offer his services to the likes of Malekith and Skarbrand without getting murdered on sight. He was corrupted and bound to the book, becoming Tzeentch's little bitch. The book also released the Lord of Change called Sarthorael the Everwatcher, who shapeshifted into a raven and started to manipulate the former wizard...
And yes, his character is somewhat "similar" to the canonical character Egrimm van Horstmann, to the point that some players confuse them.
Total War: WARHAMMER
Once the game dropped, a weird old man was introduced as the advisor to playable factions of the game. While some speculated that he is the same guy as the one in the trailer, CA rejected these speculations initially. He helped players in the creation of their own empire, battle strategy, building and upgrade of the settlements - so, normal Total War advisor, right?
Well, not really.
First of all, players noticed how the advisor appeared to help every factions... even the ones who actively hated weak 'umies and/or saw them as nothing more than food. All these suspicions were proved correct in the Chaos campaign, as after victory The Advisor will reveal himself as the corrupted Tzeentchian Wizard from the trailer, mock you... and then die like the bitch he is after Sartorael decides it is enough. For a while, his story seemed to be complete, but then...
Total War: WARHAMMER II
He magically reappeared in the second game, despite supposedly being dead. He quite similarly serves the factions that would've probably killed him on the spot, but here... he is nothing. He has no role in the plot, he is pretty much just an advisor. Dissappointment became even more evident when CA started to introduce different advisors, such as a weird monkey for the Vampire Coast and the halfling Berry Drury for Markus Wulfhart. But then...
Total War: WARHAMMER III
The Advisor became a character in the third game, once again. He reappeared, at first, as a mysterious guy in the background of those enigmatic pre-confirmation teasers with that Imperial Astronomer. Then he revealed himself in the last two seconds of the third game's announcement trailer. After that, he became the main character of the Nurgle vs Slaanesh trailer, which gave the first hints at TWW3's plot.
The Advisor was cursed by the Tome of Fates, and he actively seeks a way to free himself while also retaining the ability to get cool spells and all from the book. He found that a drop of Ursun's blood could free him from his curse, and the said bear god is currently dying in the Forge of Souls, held captive by Be'lakor for his own revenge plan.
In order to obtain his blood of a god, he decides to allied himself to different TWW3 factions (the one/s we choose, to be correct) and manipulate them to obtain the bear god's blood. After he is freed, killed or eaten he is joined by a white raven and set out for new adventures, hinting the campaign is set before game 1. Unfortunately according to the epilogue flavor text, The Advisor was yet tricked again by Tzeentch. You see, when he open and read the book (aka The Tome of Fate) for its various arcane knowledge, The Advisor's eyesight was taken by it. The white raven that was sent to guide him, was actually the same Lord of Change from the first game (aka Sarthorael the Everwatcher). According to the daemon, it and its master has yet "done with him", so he, the "Everwatcher" will be his eye instead to places where they'll be going. And just like that, The Advisor was turned into a pawn of Tzeentch for his own lust for knowledge, and to those who had finished the Chaos campaign from game 1 knew his demise.