Kairos Fateweaver

"'That's the one! The one who ravaged my FACE! REVENGE SHALL BE MINE!'"Kairos Fateweaver is a Lord of Change.

Official Canon
Being unable to see well into the future, Tzeentch had Kairos go to the Well of Eternity and forced him inside. Kairos emerged with two heads; one head speaks the truth, while the other lies. Kairos saw the entirety of the past and future at once, driving him utterly mad. Kairos is one Tzeentch's most powerful servants, commanding incredible psychic power.

Prior to the Horus Heresy, he revealed the Imperium's grim-dark future to Lorgar during his pilgrimage to the Warp, completing the primarch's fall to Chaos. Later, following Roboute Guilliman's ressurection in The Gathering Storm, he temporarily captured Guilliman's Terran Crusade, before he escaped thanks to Cypher and a surprise attack by Skarbrand.

TTS Canon
Kairos is Tzeentch's greatest lord of change, and is very proud of this. While he often doesn't reveal his intentions or feelings, his second head (which always lies) usually gives away his inner thoughts by stating the opposite.

Inquisitor Adrielle Quist attacked Kairos after asking if he was a genestealer, not caring which head answered yes.

Tzeentch uses Kairos to tell him if he's going to win his next Children's Card Game, if he's not feeling too lazy to ask.

Kairos was present at the Battle at the Gate of Khaine, leading an army of Pink Horrors, where he proved to be one of the most dangerous greater daemons of Chaos. During the battle Karios nearly single-handedly obliterated the entire Grey Knights contingent following Fyodor Karamazov ( he failed to kill both the Dreadknight and Grand Master Elirush), although he got none of the credit, as everyone present was only giving heed to Skarbrand. He later attempted to get revenge on Inquisitor Quist, however he not only failed, but nearly killed Skarbrand in doing so. Was blasted away with the rest of his daemon army when Fyodor Karamazov merged with the Star Child.

Trivia

 * Based on information given by Karios lying head, he feels the need to be validated for his accomplishments and has a deep hatred for Skarbrand.