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!
And now 8e is here, which brings good news and bad news. The good news is that our big turkey is an absolute monster! He has standard big Daemon statlines (S8/T8/WS2+/BS2+) but with an additional 20" movement and 27W!!! For defence he has a 3++ that can become a 2++(!!) with a stratagem. For shooting he has a 60" range staff that fires out 2d6 S9 AP-4 D3 shots that create flamers if a unit with 10 or more infantry models was killed by it (though that requires reinforcement points do, meh). And in melee? Oh boi. He has 6 attacks at S16 AP-4 that does a flat 6 damage against Vehicles and Monsters! What's that you hear? Oh, it's just An'ggrath crying out of jealousy. He also has psychic powers but it's the Tzeentch Daemons discipline so no one cares. He does have a +2 to deny though and he can reflect powers back to their user on a 5+ (in practice they just take a mortal wound). His final rules are one ability that makes his staff target other Lord of Changes and Daemon Princes if they get too close and if he does he blows up on a 4+.
Now that's the good news out of the way.
The bad news is that, thanks to chapter approved, he is now 1500pts (that's more than half again his original price) so good luck playing him in most games.