The mummy is sent out by the Necromancer. It is one of the five pets that can be sent out.
The mummy is very aggressive if encountered. He bites and occasionally uses his special scratch which deal a decent amount of damage. They have a slow movement speed but they have a decent attack power and amount of hitpoints. His weakness actually are Golems.
The cost to summon a Mummy is 50 souls. There is a maximum of two mummies per Necromancer.
In 6-9, when a Necromancer summons a Mummy, they appear much larger.