Some Bleak Veil Necromancers
The Bleak Veil were originally a displaced group of necromancers from Hammerfell, led by Magnifico Bahraha, that relocated to Hew's Bane. The group began to build a specifically designed royal tomb which could used as a conduit of necrotic energy for rituals. However, once Prince Hubalajad learnt of his uncle's plans, he had them sealed into the tomb—which was soon known locally as Bahraha's Gloom and the incident became the secret shame of the Royal family.
Unbeknownst to the outside world, the cult continued to survive, with its' cultists drawing lots to see who would be sacrificed for the ascension of the few. The seal which keeps them imprisoned needs to be recharged periodically and once had a entire order devoted to it. By 2E 583, the order has dwindled to a single member.