City of Heroes has 'raid content' anything like other MMORPGs is arguably the Hamidon, with the Rikti Mothership coming a close second. Originally a molecular biologist called Hamidon Pasalima, this horror underwent deliberate mutation to become a single cell of titanic size, which occasionally surfaces in the Hive zone heroside, and the Abyss zone on the villain end. Hamidon is considered to be the leader of the Devouring Earth faction.
Raids on the Hamidon used to be frequent, and were so predictable and easy that they became much more of a social gathering than a challenge. A major revamp to the encounter several Issues ago introduced new tactical challenges that made him much harder to defeat. Hamidon now had to be spawned, and the mitochondria had new powers that worked together in a frighteningly lethal synergy. Some blasts, for example, left characters unable to be healed for a period of time, a mechanic that had not been seen before - nor has it been seen since.