Fusion grenades are the Watchers' grenades. They are shaped like the thermal detonators from Star Wars.

Once they are detonated, fusion grenades emit a powerful force field in a wide area around themselves; these force fields are damaging to anything inside of them (this is probably due to the particles let off by the grenade once it explodes).

These grenades can be cooked in order to allow them to go off more quickly after being thrown, which can be very useful for flushing enemies out of cover or killing large groups of enemies at once.