Undead Creatures are neither dead nor alive. Some of them were certainly normal men and women at one time, but they have long since died, and in many cases, they walk through Tibia as mindless monsters, terrorizing adventurers and killing anyone who approaches them. The regular corpse isn't indirectly turned into an undead. Those who are turned into undead were probably manipulated by a deity to become a soldier in an endless war against the living.
Usually a deceased person to become an undeath had to become a student of Urgith. Regular persons who are not acquainted with the arcane arts of the necromancy are usually guided by Toth, the Warden of Souls to their eternal rest.
Since very few creatures are able to make themselves "undead", most undead creatures have been converted or conjured by Necromancers.
The largest group of undead creatures can be found in Drefia, where the Necromant King has fled to refuge when he has backstabbed an ancient order and created an unholy brotherhood. Because of their involvement in the plot against the Pharaoh of Ankrahmun, many undead creatures were banished to the sealed tombs under sand in the Kha'labal.
See also Undead Humanoid NPCs.
Undead Humanoid Bosses
Undead Humanoid Arena Boss
|Creature Types of Tibia|