I propose two affixes:

Vampirism: This affix can be obtained by monster only during new moon, unless you use command.

Attacking mobs with this affix with any weapon normally cause reduce their HP until 2 HP(configurable). When "vampir" has 2 hp, then he cannot be killed with any other weapon but wooden weapon or tool.
This this property may be set to don't work during day(configurable).

Lycanthropy: This affix work similar as Vampirism, but this differs in that monsters can get this during full moon, and golden gear(or silver from mods; configurable) is required to kill them.

This will force players to careful observe moon's cycles for prepared suitable gear(wodden or gold/silver sword for example) or just avoid fight.