The fire entity was a creature made completely of flame which Toa Vakama encountered and defeated on the Toa Metru's return journey to Metru Nui. Its exact nature was never discovered. Vakama saw it as a reflection of himself if he were ever to let his power run out of control. The fire entity evidently survived the encounter, since it was gone when the other Toa Metru arrived to find Vakama, who had fallen unconscious.
The fire entity was a Rahi.