All Tomb Kings have dire curses inscribed on the entrance to their tomb and repeated on their mummy wrapping, artefacts or sarcophagi. These vary a lot in their effect depending on the gods and daemons invoked by the sacred hieroglyphs. The curse is called down on anyone reckless enough to despoil the tomb or destroy the Tomb King's mummy.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
The CurseURL Copied!
If a model with the Curse rule is removed from play, then the enemy unit responsible – by inflicting the final wound, for example – will immediately suffer D6 Strength 5 hits (if the model was a Lord) or D6 Strength 4 hits (if the model was a Hero). These Wounds are distributed as for shooting hits.
In close combat, any Wounds inflicted count towards the combat result. If more than one unit is responsible for the destruction of the model with the Curse (it is destroyed by the combat result in a multiple combat due to its Unstable rule, for example), then all guilty units are affected. If the model is killed in a challenge, then only his opponent is cursed, and not the whole enemy unit.