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The Quarry - The Loop
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You see Mahatheb.
Notes
You can read more about Mahatheb in this book. This boss spawns quite rarely instead of a Vampire in the last chamber of the Forgotten Tomb on floor -6.
Abilities
Damage Taken From Elements
Physical 

Death 

Holy 

Ice 

Fire 

Energy 

Earth 

Location
Bottom floor of the Forgotten Tomb in a Varying Monster Spawn in which the common creatures are a Vampire and a Bonebeast.
Behaviour
Unknown.