The Undead Talisman is an accessory that provides 15 defense against undead monsters. If the defense provided against them brings the damage of said NPC to 0 or below, it will make you immune to that NPC. It also provides immunity to the Broken Armor debuff.
To craft, it requires: 20 Undead Shard, 5 Gold Bar, 5 Silver Ore, and 30 Cobweb at an Anvil.