The Advisor

"Your empire dominates all. Could you rival the gods themselves? I think not, fool! (Evil Laugh)"
- – The Advisor after the end of Warriors of Chaos campaigh
"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) the major plot character in 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 trailer to the first game.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 Tome of Fates, which, quite predictably, turned out really bad. 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 raven and started to manipulate former wizard...
And yes, his character is somewhat "similar" to a 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 advisor appeared to help every factions... even the ones who actively hated weak 'umies and/or saw them as nothing more than the food. All those suspicions were proved correct in the Chaos campaign, as after victory The Advisor will reveal himself as corrupted Tzeentchian Wizard from the trailer, mock you... and then die like a 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 kill him on 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 this weird monkey for the Vampire Coast and halfling Berry Drury for Markus Wulfhart. But then...
Total War: WARHAMMER III
The Advisor became a character in the third game, once again. Seems like that Chaos campaign from the first game is completely non-canon now. 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 third game's first trailer. After that, he became the main character of Nurgle vs Slaanesh trailer, which gave the first hints at the overall TWW3 plot.
Turns out, The Advisor is still a slave of Tome of Fates, and he actively seeks the way to free himself while also retaining the ability to get cool spells and all from the book. He found that the drop of Ursun's blood, who is currently dying in the Forge of Souls while Be'lakor is using him for his vengeance plan, can do this. Due to that, he decides to venture to different TWW3 faction leaders (the one/s we choose, to be correct) and manipulate them into finding Bear God so he can get his hands on the blood. Game still wasn't released, so what happens to him after that is a mystery.