Aetaos'Rau'Keres is the most feared and powerful Lord of Change in the immaterium and is also one of the most powerful daemons in existence. Keres is insane, even by daemon standards, due to Tzeentch shackling him with numerous types of warpfuckery, to the point where he feels nothing but utter rage. Such is his power and insanity that only apocalypse cults dare summon him, because once Keres enters realspace; he will murderfuck everything on the planet he is on, neither caring if he's killing loyal cultists or the enemy, until nothing is left. In short, he's a paranoid and unfathomably pissed off exterminatus on legs (and wings).
The kicker to him is that Keres' true purpose in Tzeentch's grand schemes and his origins are his most closely guarded secrets and nobody but Tzeentch know what they really are. If anyone or anything comes close to finding out the truth about him; he will personally appear before the seeker and obliterate them and the planet they are on, just for good measure. Keres basically takes the concept of "no witnesses" to it's most illogical and hilarious extreme.
On the tabletop, Keres clocks in at 999 points, Tzeentch's sacred number, and has ML4. He has a slew of special rules, including the ability to bounce back psyker powers, summon daemons, and his trolltastic "Boon of Chaos" template ability which forces units under it to pass a toughness test or be turned into a gribbly mass of tentacles. He can also reduce a models attacks, by 1, to a minimum of zero, he can deep strike, can literally turn an entire squad of gaunts into 30 of those things, can just summon horrors for free, and has a monstrous statline of WS/W/I 9! He also counts as an Icon of chaos!