In ancient times, Sochen Cave Palace was used as a ground for spiritual training by monks and ascetics. Here they purified their hearts, yet the corruption they washed away pooled, becoming the black soil out of which the Ahriman is said to have grown. The coalescence of discarded corruption hates those with pure hearts, and attacks with vengeful animosity. For this, the Ahriman is considered an enemy by most religions of Ivalice.
Ahriman mostly attacks with magick, but it will also cast Doom. Ahriman will also cast Immobilizega from time to time. When around 40% of its HP is drained, Ahriman will start to use Divide, splitting itself. The replicas have around 700 - 1,000 HP, and are capable of attacking. The real Ahriman will appear slightly ghostly while the replicas appear solid. The party cannot hurt Ahriman with most magicks while his replicas are out.
With Thief's Cuffs one has a decent chance of getting a Death Powder. Two Death Powders are required to make a Ultima Blade, and they are hard to come by. The Ahriman replicas can also be stolen from, for a potentially unlimited supply of Death Powder.
Shell is a great asset and mastered Remedy Lores are a great help. A good gambit set is Ally:any=Remedy and healing fast plays an important role.
If the party are having trouble spotting the genuine Ahriman—it can sometimes get crowded—one should try setting a Foe: highest HP/highest max HP gambit, and one will always target Ahriman (unless he teleports across the room). Overall, Doom is the greatest burden while fighting Ahriman, but with enough Remedies, this can be an easy fight.
An easy strategy to spot the real Ahriman is to cast Regen on him when the fight begins, as he is an undead enemy, the Regen will slowly dwindle down his HP. Once he starts dividing the real Ahriman will still have Regen cast upon him, whereas the clones will have no buffs. The party can use Quickenings on the genuine Ahriman when he is bunched together with his clones to make for an easy fight.
In Final Fantasy XII Ahriman is a ghost, rather than a flying one-eyed demon, differing radically from past incarnations. Ahriman, along side Ghosts, is the only Ghost-type enemy in Final Fantasy XII to have an eye by its 'chest'; the other ghost-types have either a gem or some sort of 'heart'.