The Overloader is an expensive material which can be used in the creation of Swarm Boss summoning items, which summon 10 instances of a given boss. It can be bought from Mutant and Abominationn in Expert Mode. By stacking multiple swarm boss summoning items and using them, approximately 10 bosses will appear for every summoning item in the stack. Only about 10 bosses will be active at once, and as bosses die, other bosses will spawn in to replace them until the intended number of bosses is defeated, based on the number of summoning items used.
Swarm bosses are intended to be fought in groups of 100; however, the amount of bosses summoned actually seems to be random. Tests have had the number of bosses vary between 70 and 127 bosses per swarm. It's highly recommended that you greatly exceed the power of the boss you're fighting before you call the swarm in, because even weak bosses can overpower players in sheer numbers.