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).
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 bylut 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 (turned a Slaneshi sorcerer into a beanstalk) and his army seemed to win the subsequent battle of the 3 sisters. However, during the fighting, some dwarfs exploded him inadvertently while emerging from a tunnel just as he wa 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.