Immense Flesh Corpse is a level 52 Mini-boss found in Nightmare Dungeon and Bad Dreams. Like all bosses it has a longer respawn time then most of the monsters and there is only one in each dungeon at a time.
Immense Flesh Corpses have 120,000HP and 3,000 MP. They do not seem to use their MP to cast any spells. Instead they use melee attacks to do 1450 damage. They have 990 defense and can drop any of the following items.
Demonic Essence at a 0.09% drop rate
Cocytus at a 1.5% drop rate
Opaque Katana at a 1.7% drop rate
Bloodbone Ring at a 2% drop rate
Level 45 Armor at a 5% drop rate.