Orcus grants his rank-and-file cultists the Undying Soul trait, and his cult leaders can gain the Aura of Death trait.
Undying Soul (Recharges after a Short or Long Rest)
If this creature is reduced to 0 hit points, it immediately makes a DC 10 Constitution saving throw. If it succeeds, it is instead reduced to 1 hit point.
Aura of Death
This creature emanates a deathly aura that extends 30 feet in every direction from its space while it isn't incapacitated. The aura is blocked by total cover. While in the aura, the creature and any friendly Undead are immune to the frightened condition and have resistance to radiant damage. Enemies have disadvantage on death saving throws while in the aura.