Provide some ways for Vecna to come back after being defeated:

1. Allow arch-liches to grow up into Vecna if Vecna does not currently exist.
2. Any humanoid caster that acquires the Hand or Eye of Vecna should instantly become Vecna. Covetous liches should also covet those artifacts.

Arch-liches sometimes generate with potions of unholy water in their inventory.

There should be one major monster of each alignment that is ungenocideable: Archon for lawful, deep ettin for neutral, arch-lich for chaotic.