Potion of Immortality in Heroes 4

When a hero who drinks a Potion of Immortality dies in combat, he will instantly return to life. This potion can be used before a battle, and only wears off when the hero dies. It resurrects the hero only once.

A paranoid enchanter named Staad spent several years visiting seers and soothsayers before he came up with the ingredients for this potion, only to discover that he couldn't use it because it gave him a terrible rash.