Dahaka, also known as Azhi Dahaka, Zohhak or Zahhak, is the three-headed dragon of destruction and terror spawned from Angra Mainyu to wreak havoc on the world, first mentioned in the Avesta, the main collection of Zoroastrian religious literature. After being defeated by the heroic Fariedone, he has been sealed away by Ahura Mazda. However, Azhi Dahaka would eventually escape his prison under the mountain Demāvand and lead the forces of evil against all of creation, destroying one third of it before being defeated again by another hero, this time decisively.
It's hard to tell if the Avestan Dahaka was supposed to be a dragon with human traits, or a monster-like human king; only the fact he had three heads is clear. In later versions of the tale, Dahaka is, however, directly said to be a human ruler of a fictional land often identified as Babylonia. After the yazatas Anahita and Vayu refused to grant him his wish of depopulating the world, his reign was influenced by a series of advisors all of who were Ahriman in various disguises. He's depicted in art as a man with two snake heads on his shoulders. The Denkard describes him as a powerful sorcerer and skilled artisan whose crafts made people abandon the teachings of Zoroaster in favor of idolatry. In many Pahlavi texts, Dahaka is identified as the source of "bad religion" and "fake law," and one of the seven worst sinners in the history of the world (a similar epithet is also applied to another Zoroastrian dragon, Azi Sruvara).
Some Zoroastrian texts present Dahaka's period of cruel rule as the lesser evil, arguing the alternative was an eternal reign of Aeshma, impossible to vanquish by a human hero. He was also known as Zahhak the Snake Shoulder.
Vritra and Dahaka are likely derivatives of the same creature from Proto-Indo-European mythology, but the location of Dahaka's fortress in later tales shows additional influence from Mesopotamian mythology, especially from the myth of Marduk's struggle against Tiamat.
Azhi Dahaka is a special enemy located in the final segment of the last dungeon Monado Mandala. He can be first encountered when the party opens the gate leading to the final branch. It is initially invincible to anything including non-elemental attacks and this battle is retreat-able. The only way to wear off its invulnerability is to summon and equip the Star ArcanaPersonaFariedone. At the start of the battle both entities will initiate a Persona-demon talk and both will become furious (the one using Fariedone will be uncontrollable during the battle and no cure is effective). After defeating Azhi Dahaka, it will become a regular enemy that appears in the random encounters of the same area without the invincibility.