Ahriman is an enemy in Final Fantasy IV: The After Years. It is fought by Cecil Harvey, Rosa Joanna Farrell, and Cid Pollendina during Baron's invasion, in a storyline battle.
The Ahriman is not a great threat. It will cast Doom on a party member, then cast Haste on them to speed up the countdown.
Rosa should cast Slow on the Ahriman, and then the player can use whichever Band abilities they prefer to kill it.
Angra Mainyu in the Avestan language and Ahriman in Middle Persian is the Zoroastrian idea of the devil, meaning "destructive spirit."