Death Spiral

The Death Spiral, also called the Spiral of Death, is a ritual in Overlord performed by a group of magic casters, and the practitioners known for it are members of the death cult Zurrernorn. It works off a property of the undead: stronger undead spawn on their own wherever weaker ones gather in numbers, and stronger ones again where those are packed together, so the cycle feeds itself. The rite makes deliberate use of that, spawning ever more powerful undead, and its real aim is to collect the negative energy the process generates and seal it inside the caster, who can be turned by it into a powerful undead such as an elder lich. Carried out properly, it is held capable of turning a city of the living into a city of the dead, given the number of living sacrifices it demands.