Aekold Helbrass was a Chaos Champion of Tzeentch introduced in the 4th edition of Warhammer Fantasy, and was then promptly forgotten (getting a mention on the map in the 8th edition Warriors of Chaos army book but not appearing until The End Times).
Aekold was originally just a bored nobleman seeking purpose to his life, becoming a knight in the Order of the Jade Griffon. Eventually he became disatisfied with how he believed the Church of Sigmar was unable to change the world for the better, leading him to join the Brethren of the Golden Eagle, a revolutionary society that was a front for a Tzeentch cult. Although Aekold progressed up the ranks of the cult, eventually it was exposed and Aekold was disowned by his parents and fiancee, forcing him to flee north. In Troll Country, he discovered an ancient monolith to Tzeentch protected by a minotaur; upon slaying the minotaur, he discovered a carving on the monolith that was centuries old yet depicted himself.
Aekold went further north to the Chaos Wastes until he found a mirror-portal, guarded by a reflection of himself that represented what could have been if he remained loyal to Sigmar. After slaying his own reflection, he had one last moment of regret for his deeds before realizing that he was too far gone to go back and entered the mirror-portal, returning transformed into the Champion of Tzeentch.
What separated him from other champions and lords of Tzeentch, was that he wasn't a wizard, and his special ability given to him by Tzeentch: the Breath of Life. See, wherever Aekold wandered, he would make the land spring into life, as grass and flowers would grow around the area he walked. This ability also allowed him to heal those in the threshold of death (presumably annoying Nurgle greatly), and make wooden objects start taking root again.
He fell during the End Times, dying in battle against Felix Jaeger. Before this though, he appeared to have been the main catalyst for the destruction of Kislev. He lost control of Praag to Throgg but went on to win the battles of the Lesser Tobol and at Kislev City. Finally he went on to use his Breath of Life powers to breach the Auric Bastion with ease (turning a Slaaneshi sorcerer into a beanstalk in the process) and his army seemed to win the subsequent battle of the 3 sisters, even managing to kill Vlad von Carstein (as usual, Vlad got better because of his ring). However, during the fighting, some dwarfs exploded him inadvertently while emerging from a tunnel just as he was beating Felix Jaeger.
The way the dwarfs and Kislevites refer to him in Kinslayer, you would think he was the Everchosen or a top lieutenant, not a character returning from the narrative wilderness.