Kairos Fateweaver is one of the more powerful Lords of Change in the service of Tzeentch, known as his Oracle. His name derives from an Ancient Greek concept of the key and propitious time for an action to be taken. While χρόνος (chronos) referred to time as a linear series of events, καιρός (kairos) had "a qualitative, permanent nature." Very apropos of the warp.
While Tzeentch knew everything there was to know about the past and present, he was unhappy that he couldn't get a perfect view of the future. In an attempt to overcome this issue, he went to the Well of Eternity at the center of reality. (How much of this is intended to be absolutely literal is absolutely unclear, but the 40k universe is a strange place.) However, because Tzeentch is kind of a massive bitch, he didn't want to go in himself and sent his Lords of Change instead. The problem: none of his servants ever returned.
Fed up with this, Tzeentch eventually grabbed Kairos and tossed him into the well. He survived, but emerged incredibly aged (which for a creature of the warp immune to the passage of time is saying something) completely insane (even by daemonic standards) and with a second head (resemblance to the Imperial Aquilla is ENTIRELY coincidental...). He knows everything from the past and the future and when asked a question, one head will tell the truth, and the other will tell an equally believable lie.
It's possible that the Imperial Aquila is a representation of Kairos. *BLAM* Heresy!
During Lorgar's quest for knowledge in the Eye of Terror, Kairos appeared to him and gave Lorgar a choice, saying that the Aurelian could either seek revenge on Roboute Guilliman for the destruction of Khur, but ensure that Chaos would lose the upcoming war, or put aside his personal desires and work for the big four, and ensure their eventual victory. Aurelian notes that in this single instance, both of his heads were telling the truth. He also disguised himself as an entirely different oracle (with a little bit of a shout out to H.P. Lovecraft) in order to trick Ahriman and the Thousand Sons into regathering Magnus's shards and binding them together in a way that Tzeentch preferred (oddly, getting Lucius the Eternal to help out, who was just kind of there, doing his dueling thing and fucking off from his chapter to hang out with the Thousand Sons for a little while.) It's been implied that Kairos tried repeatedly to influence the Lion in the time he wandered the forests of Caliban as a youth; unfortunately for Kairos however, he really couldn't have picked a worse primarch to try this on, and the Lion being the Lion each and every time shut him the fuck down until big bird just gave up. Centuries later he tried overtly attacking the Lion and his fleet during the Thramas Crusade, and that went even less well.
He's also part of the ship, "Accursed Eternity" that was formerly a space marine battle barge and the one who taught Blood Drinkers the blood ritual.
He was tasked by Magnus and allied with the Red Corsairs to capture Guilliman by binding the primarch with chains made of his own guilt, anger and disappointment. He initially succeeded but was immediately fucked over by none other than Cypher, who led a troupe of Harlequins and a bunch of Khornate daemons led by Skarbrand to fuck him right up his daemonic cloaca.
The End Times
He tells Archaon that he will be killed by someone "who is not mortal", but Archaon is like fuck that shit and tells him to attack Averheim where Valten resides. Like all things Tzeentch related, he failed and had his heads cut off to summon a much useful servant, a greater daemon of khorne Ka'Bandha.
Truth or Lie?
To get a straight answer from what is effectively two creatures where one tells the truth and the other lies is tricky but doable. provided you restrict yourself to true/false questions. Karios's heads work much like the Knights and Knaves logic puzzle made famous in Labyrinth. You can pick either head and, for some true/false question Q, ask, "If I asked you Q, would you answer True?"
|You picked the Honest head.||You picked the Lying head.|
|The answer to Q is actually True.||Yes||No (lie)|
|The answer to Q is actually False.||No||Yes (lie)|
Unfortunately we're not done. Although not explicitly confirmed in canon, which head lies and which head tells the truth will probably change with each question, unlike in Knights and Knaves, so you should not determine which head is lying for this question and then use that knowledge to change your next question. For example the Knight and Knave puzzle is often framed like one will lead you home the other to death hence why knowing which one lies is so important, if after asking the question the Knight and Knave could switch roles, then you have no way of knowing whose lying and therefore knowing which one will take you home/doom you. Asking Kairos questions for which there is no false answer, such as "If 1=2, then what is the Emperor's favorite cheese?", or no true answer, such as "How many inches do I weigh?", making him incapable of lying or telling the truth, may result in the sort of paradox that leaves you thoroughly Tzeentched.
As you can imagine attempting to outsmart an insane, immortal, all-knowing daemon isn't the best of ideas. Unless Kairos is fully compelled to answer you truthfully (like in the case of Lorgar); he might just flip the table and turn you into a gribbly mess of mutated flesh as his way of refusing to put up with your shit. In fact, since Kairos has already seen the past and future it's highly likely he has already foreseen your attempts at trying to get a straight answer out of him and prepared his answers beforehand. Thus, Kairos is less "knights and knaves" and more "Hardest logic puzzle ever by George Boolos". In addition to one liar and one truth-teller, add in a random element (traditionally a third character who answers at random, and who is indistinguishable from the others aside from his answers; in Kairos's case, he probably swaps heads), and add onto that they deliberately answer in a language you don't speak (The three gods riddle), and of course they not only know exactly what kind of question you're going to ask but will also make sure that both their responses sound perfectly true with no way to tell which one is false, and further add that if they don't like your question they are going to kill you. And then laugh. And then display your corpse at the museum of failure wearing a silly hat.
Basically the only way to get an answer out of him IF you have a way of phrasing the questioning such that you bypass the whole which head lies and which tells the truth thing, is if you are strong enough to impel him to answer your question: and in 40k the number of characters that can do that are on a very short list, which spoiler: you are not on.
On the Tabletop
For 40k 9th edition, we are still waiting for a proper codex tho
Kairos now has the exact same point cost as a regular Lord of Change, so comparisons are pretty straightforward. The main thing you loose is one point of WS (the LoC has 2+) and some deal of customization, but for everything else Kairos is straight up better. To start with, the LoC comes equipped with a Staff of Tzeentch that has S=6, AP=-2, D=3, whereas Kairos comes equipped with the Staff of Tomorrow that gives S=8, AP=-3,D=D6, and both weapons turn slain characters into a Chaos Spa... One of those fucking things, the LoC has to pay 5 points extra to get that weapon profile with a Baleful Sword, but this sword works like a power fist in that it substracts 1 from all hit rolls and its just a +1, instead of a +2 to strenght.
Similarly, Kairos has a Smite Range of 36", whereas a regular LoC needs to be equipped with a relic to increase it to 30" from its default 18". The lord of change knows two spells from the tzeentch discipline and can increase the known amount with relics and stratagems, but Kairos knows all of them. Finally both the regular LoC and Kairos have a bonus to psychic casts that start at +2 and go down as they take wounds, but the LoC can cast and deny two times per phase whereas Kairos can cast and deny 3 times. The biggest reason reason you would want to use a LoC over Kairos is have a tankier bird with The Impossible Robe relic that gives it a 4+ invul save (3+ after Ephemeral form) and a good warlord trait, like Incorporeal Form to give it a "disgustingly resilient" reduction of wounds taken or Daemonspark to make your horror blobs terrifying.
Whatever the case, Kairos will make sure to make your opponent dispair during the psychic phase and he'll be able to ahnihilate one or two vehicles with his flying movement and melee attacks that hurt like lascannons, just be sure you have a way to prevent HIM from getting sniped by lascannons first