Kairos Fateweaver
Kairos Fateweaver is the most powerful Lord of Change in the service of Tzeentch, known as his Oracle.
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. 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. Kairos survived, but emerged incredibly aged (which for a creature of the warp immune to the passage of time is saying something) completely insane (for a daemon) and with a second head. 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.
During Lorgar's quest for knowledge in the Eye of Terror, Kairos appeared to and gave him 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.
Truth or Lie?
To get a straight answer from Kairos is tricky but possibly doable. Karios head's work much like the Knights and Knaves logic puzzle made famous in the Labyrinth. Typical versions of the puzzle have you try to figure out if you're talking to a "knight" (truth), or "knave" (liar), often with a single question. However such puzzles have well thought out solutions and are not considered even 'puzzles' to the Lords of Change. The issue with Kairos is that which head lies and which one tells the truth likely vary from moment to the next so that even if you did ask the golden question and managed to figure out which head was the Knight and which was the Knave, it would not even apply from one question to the next. This actual makes the puzzle closer to the The Hardest Logic Puzzle Ever where there are three gods, one truth, one lies, and one random who you don't know which words they say are yes and which are no. Effectively with Karios you have a truth head, a lies head, and then the heads may randomly swap, but you don't know if they have done that meaning over the course of two or two hundred questions you still can't be sure which head, and oh he might not even be speaking a language you understand. The solution is that you need one question which both gives you the answer to what your looking for, and Identify's which head is the "knight" and which is the "knave". The more open ended the question the harder it is to figure out which head is truth and which is lies. If your just looking for confirmation of a item's location you could ask something along the lines of "would your other head say the magguffin was on earth?" (If the magguffin was on earth) the 'lie' head would say no as the 'true' head knows the maggufin's on earth. The 'true' head would say no as the lie head would deceive you and lie about the position. So when both heads say no, the magguffin is there. But if you have the Maggufins location down to just a handful location out of the billions of planets in the galaxy there's no reason to resort to Fateweaver to figure it out since you could just spend a few more dollars and scout the locations out your self rather then the much larger risk of consorting with the most conniving type of daemons. The case where things that you must resort to Fatewaver for a solution are so rare, and would require questions of such sufficient complexity makes wording the question to both answer what you want to know and to figure out the truth head from the lieing head incredibly tricky. Further if you are such a tricky smart person, then Tzeentch may just not even let you talk to Fateweaver, or just kill you the second you do figure out because of course he would.
