I love special characters
About Ahriman, here are my propositions:
- he is mainly a sorcerer and his CAF is really high, +2 or +3 should be enough for a Tzeentch unit
- the inferno bolter is OK but you could let it as a normal bolter and add the special ability of creating pink horrors to the staff with the following abilities:
> Black Staff of Ahriman: once per turn, place a normal barrage template within 35cm and LOS of Ahriman stand, any infantry stand at least half under the template is hit on a 5+ ignoring cover modifier. This is an ethereal psychic attack and no armor save is allowed (but psychic save still works). Units killed by this attack are replaced by a pink horror.
> Keeping the Negate (Psychic save) of 3+ and the +1 chaos power card is fine.
- about the Flying disc, it's a good idea but Ahriman will not be an infantry stand anymore so it will be easy to shoot him if there are no flying disc or any other skimmer in front of him. Maybe a command rhino could do the job.
Maybe some restriction could be added, like: can only be taken if at least 3000 points of Tzeentch force is fielded.
With this "reduction" of power, cost could be reduced. 250 points for a special character is really high, usually, when I create a profil for a special character, I try to match ~150 points for a single unit and ~250 points if an escort is provided with the stand.
