Exiled from the planet of the sorcerers, Ahriman now strides the galaxy seeking out lost lore to better master chaos. In past editions of the chaos space marine codex, Ahriman was always a strong psyker, but not the ultimate psyker. With the new chaos space marine codex, this changes: Ahriman is now the psychic terror that he deserves to be, second only to Magnus the Red himself perhaps.
Ahriman is now a level 4 psyker and can cast up to 3 witchfire powers every turn. That is quite a potent psyker by comparison to a number of other HQs in alternative codexes. That said, there is now no automatic gain of powers like bolt of Tzeentch. He has to roll like all other psykers. But Tzeentch gets access to the good powers of the codex: firestorm, doombolt and breath of chaos. In addition there's a chance of getting something good from the other disciplines (telepathy, pyromancy and biomancy). Invisibility might be a good roll for Ahriman for instance. A good division of rolls might be 2 from Tzeentch and 2 from Telepathy (and select the primaris psychic shriek if you don't roll invisibility!).
On the whole, Ahriman is a psychic firebase. And unlike a heavy support option, he can be highly mobile. So, don't hold him back: stick him in to a rhino with some other marines (regular ones with some special weapons perhaps; or even rubric thousand sons marines in a pinch) and start to channel all that potent psychic potential to some good use.
The downside is his points cost: he could do with a longer range than the "typical" 24inch or less range that many powers have for his price tag. In this regard, a regular sorcerer might be a better fire support officer when placed in a rhino with another squad. That said, he won't have 3 witchfires going off every turn, but with a few maledictions and blessings, that may not matter.
Like Huron, Ahriman's warlord trait is Master of Deception. D3 infiltrators is nothing to be sneezed at with the right combo and will certainly help other units get in to close combat more effectively.
And lastly, although Ahriman won't be getting in to too many close combat fights, his force weapon (the Black Staff) is reasonably potent too. Just pray that he doesn't become either a chaos spawn of daemon prince: he's more valuable as a fire support base in my opinion, so we only want him in melee as a last resort. A daemonic pink herald of Tzeentch on a chariot or disk might be a better option all round really (at least until we learn of a new daemons codex).