The Cleaner

The Cleaner is a black level 73 elite demon. Although he looks like a black Dreadlord he appears not to be one.

Abilities
He can hit for 4500-5000 on a lvl 70 cloth wearer and is said to be immune to all damage.

Source
The Cleaner spawns when someone besides the hunter attacks Simone the Seductress during the level 60 hunter quest, and proceeds to kill the interloper and the hunter while Simone despawns. He can also be triggered by attacking Simone in her troll form, however you need to have over one player or mob on her aggro table, otherwise she will simply kill you with her Fool's Plight.

He also spawns if someone interferes with a priest doing quest.

Easiest way to currently spawn this mob seems to be to pop Army of the Dead and then aggro one of the demons.

It seems that a Cleaner spawns whenever damage is dealt by an outsider source, therefore more than one Cleaner can be spawned at the same time.

Quotes
"You dare interfere with this being's testing? The battle must be fought alone! You shall all pay for this interference!"