You see Deep Terror
Notes
It is the first boss you will encounter during the Hero of Rathleton Quest. It will spawn when you kill the four Tentacles of the Deep Terror on the quest. Like the other bosses, you will not loot anything unless you have first slain Professor Maxxen. Thus you will not loot anything on the first kill.
Abilities
Melee (0-945+)
Self-Healing (0-921+)Damage Taken From Elements
Physical 
Death 
Holy
Ice
Fire
Energy
Earth
Location
His lair in the Underground Glooth Factory in Jaccus Maxxen's Dungeon
Behaviour
Retargets to whoever deals the most damage.
Strategy
Trap it with knights and wild growth runes so that it cannot reach the shooters. All shooters should use their strongest spells to bring it down as fast as possible since it heals a lot and you have only 10 minutes in the boss room.