From Dungeons of Dredmor Wiki
It's a Dread Collector. It likes pulverizing things, and it hates Criminal Scum like you.
Latin Taxonomy: daemonus collectus
Base level: 20 (Special)
XP Value: 20*
Skill Levels: Fighter 10, Rogue 2
AI Aggressiveness: 10
Spells: 25% chance to cast the following:
Dread Collectors are only seen when Brax has been angered, either by being attacked or stolen from. Once the Dread Collectors begin to spawn, the only way to stop them is to kill every Brax on that level. Even then, the Dread Collectors never despawn, and will continue to be aggressive and chase down the character. There is no known way to appease Brax or the Dread Collectors.
Even though the XML file indicates no experience is gained for killing a Dread Collector, they are actually worth 20 experience points each. "Criminal Scum" refers to everybody's favorite guard from Oblivion.