Ur-histachii look like an undead counterpart of a living histachii. They share the same physical characteristics with some minor differences. They were often covered in unhealed open wounds, in which mold grew and maggots lived. They had a distinct odor, a mixture of acid, decay, dust and rust.
Ur-histachii mindlessly attacked any non-reptilian within range, using their fangs and claws in a charging attack.
Despite being undead, ur-histachii retained the same senses of when they were alive, including infravision with a 90 ft (27 m) range. Some ur-histachii were bestowed the ability to enter into a berserk state up to three times a day. Ur-histachii had an immunity to charm, cold, hold, and sleep-based spells. They also were immune to poison and death magic. They were vulnerable to blessed or holy water.
Ur-histachii were created (possibly exclusively) by the Cult of the Dragon cell and their yuan-ti allies in the catacombs beneath Hlondeth. Their main use was to guard vital stockpiles and standing guard outside important meetings. Since they were undead and did not require oxygen, they were also used to guard important chambers that were sealed. They were not allowed near the brood chambers due to fears they may "forget" their duties and eat the young yuan-ti. Other yuan-ti avoided interacting with ur-histachii as much as possible.
Ur-histachii were created in a hideous ritual overseen by necromancers of the Cult. It was a costly ritual, involving the use of several expensive components and was physically draining on the necromancer overseeing the process. The ritual lasted up to three days and a maximum of eight histachii were ritually slaughtered and transformed per session.
Being undead, ur-histachii did not required food or water to survive. Despite this, they tended to chew on carrion and small vermin when idle.