Ignus was both a powerful mage and an unstable pyromaniac, never a good combination. It is implied that he learned his trade from the player character, The Nameless One, in a previous incarnation when he himself was a mage. On the day that Ignus finally went totally insane, he decided to improve the city of Sigil by burning it to ashes. Despite his incredible power, he was stopped from this campaign of destruction by all the other mages in Sigil joining forces to subdue him. Mages being big on ironic punishment, they decided to execute him by opening a portal inside his body to the Elemental Plane of Fire, a universe consisting entirely of flames. However, much to their surprise, not only did Ignus manage to survive, he revelled in the eternal flames that engulf his body without ever entirely destroying it.
When The Nameless One's current (read: playable) incarnation finds him, Ignus is floating in one spot, seemingly comatose in his ecstatic enjoyment of the fire, and is used as a conversation piece at a bar named after him, the Smoldering Corpse. Pouring water (from a magical flask that never runs out) onto him fails to quench the flames, but it does get Ignus' attention long enough for The Nameless One to convince him to join his party. Ignus is still a powerful mage; even if every single one of the spells he knows when he joins the group revolve around setting things on fire, he is capable of learning and mastering any offensive spell in the game.
Although he is a good set of hands in combat, Ignus is very untrustworthy, due to being completely insane. If he is in the party at the end of the game, and the player is of the Good alignment, Ignus will easily be convinced by The Nameless One's adversary, The Transcendent One, to turn his flames against him. It doesn't take much convincing, really, he only follows the player in the first place because adventuring gives him ample opportunity to do the only thing that brings him joy.
"Ignus wishes to burn..."