Boom Fiends are gremlin-like enemies that carry bombs. When approached, they will run at the Skylander all the while laughing maniacally. When they get close enough, the Fiends will light the fuse on the bombs they're holding and quickly attempt to explode on the Skylander. If successful, the explosion will inflict a lot of damage, and the Boom Fiends will destroy themselves in the process.
They also appeared in the Ground Defense online Skylanders minigame.