While Obsidian has not yet confirmed the whether this effect is in NWN2, it is a side effect of the Brimstone Blast invocation which has been confirmed.
A character that has been set on fire is immolating. Each round, the subject & its equipment take 1d6 fire damage. A new Reflex save vs. DC 15 is allowed each round to put the fire out. +4 bonus for rolling on the ground or having useful help. The fire goes out automatically if the subject is doused with water, jumps in a lake, etc.
Note: Getting set on fire by brimstone blast does 2d6 damage each round.