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.
TTS Canon
Inquisitor Adrielle Quist attacked Kairos after asking if he was a genestealer, not caring which head answered yes.[1]
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.[2]