Fateweaver, Oracle of Tzeentch

In a nutshell: Hello Dolly! One of four major game-making units in the Codex. He's pricey as all hell, but giving everyone in range a re-roll for any save (except FNP, sorry Nurgle!) is probably worth most of the price you pay. He is a MC that flies (Daemons need that), and he has all the Tzeentch toys except Warpfire. He is however primarily utility, not meant for CC or anything and using two of four HQ spaces is a hidden cost you must weigh when considering whether to bring him. He is totally useless in an army not built around him, so don't bring him unless you plan to use him as your lynchpin.

Pros: 3+ re-rollable invulnerable save. BS5 for his shooting attacks, and can fire gaze, bolt, and breath at 3 targets (ok and boon too if you really want to).

Cons: Grey Knights with Pyscannons. Chaos Sorcerors with Lash. CC. Mass torrent of fire. Even a 3+ re-roll doesn't last against guided-doom war walkers with scatter lasers. Or anything that can get close and rapid fire, T5 isn't strong enough. Marine Librarians with re-roll invulnerable saves counter your ability, and that's a sad day. You'd think this guy would get psychic defenses, but apparently the master of magic gave that right to Khorne and Marines (oh and Eldar).

Tactics: Sit back behind a wall of bloodcrushers, or some shooty Tzeentch Heralds and DP's. Own many many armies. Fear Land Raider armies.

Final word: He's got to be the heart of your army tactics. Don't mess around with this one, just make him integral and be done with it.