Unhealthy undead

Often any item, skill, spell or whatever that would revive otherwise dead person usually has completely opposite effect on undead, that is, they completely destroy them. Tends to be especially effective in boss battles.

This would also imply that killing anything beyond 0 health (to negative health) would cause them to turn into undead, but this seemingly never happens.

Occasionally the undead can be healed for real, but this is often accomplished by some form of negative energy spell or similar.

There are also games which allow enforcing undeath on anyone (called "zombification" in some older console RPGs), which either the opponents or the players can quickly follow with revive which effectively kills anyone for good.