Elixir of Life

One of Nagash's chief successes was the creation of the cursed Elixer of Life. With it, Nagash finally unlocked the secret of eternal youth. He then allowed Arkhan, his trusted vizier, and his other principal lieutenants to imbibe the elixer, granting them immortality and incredible strength but, unable to recreate the potion themselves, they became little more than slaves to Nagash's sinister will.

Source

 * Warhammer Armies: Tomb Kings (8th Edition)
 * pg. 13