IX Undead Killer is a support ability learned by Zidane Tribal, Adelbert Steiner, Freya Crescent and Amarant Coral. When equipped, the wielder deals 1.5x the normal damage with physical attacks against all undead opponents. Killer abilities do not stack in case an opponent belongs to more than one category; for example, Dracozombie is both an undead and a dragon, but it gives no additional benefit to equip both Killer abilities.
Undead Killer takes two Magic Stones to equip. Zidane and Amarant learn it from N-Kai Armlet, Headgear and Ritual Hat for 45 and 10 AP, respectively, and Steiner and Freya learn it from Silver Gloves and Plate Mail for 30 and 20 AP.
XI Undead Killer is a job trait in Final Fantasy XI. It occasionally causes members of the Undead family attacking the player to freeze in terror, similar to Paralysis. It is obtained by Paladins at level 5, and for Blue Mages via equipping Bludgeon and Smite of Rage. This can be done as early as level 34.