Undead Fortitude is a passive feature that prevents the user from being killed, unless hit with a Critical Hit or 
Radiant damage.
Description
If the killing blow wasn't a critical hit and wasn't radiant damage, this undead comes back to life with 1 hit point.
Condition: Undead Fortitude
- When reduced to 0 hit points, regain 1 hit point instead (unless the damage was radiant or caused by a critical hit).