In the DLC Tiny Tina’s Assault on Dragon Keep there is a random spawn NPC named Mr. Miz in the Lair of Infinite Agony who will give you the quest 'The Amulet'. As stated, he is a random spawn so he will not always be in the area. If you go there and dont see an exclamation point quest marker on your map, quit out and continue until you do.
Once you find him and accept the quest, he will offer to sell you an amulet he claims is of great use (it actually does nothing in particular and is thought to be a reference to The Pendant in Dark Souls) for a lot of money. For me, at lvl 40 and on normal, it was roughly 175K. You are given two choices:
-Purchase the amulet: If you choose to buy it, he will give you the amulet (which counts as a relic) and youll complete the quest. Simply going into your inventory and equipping the 'Mysterious Amulet' will unlock the achievement.
(Note: If you buy the amulet but dont want to lose your cash for good, buy it and wear it so the achievement pops then dashboard and close the game manually without using the save and quit option. Completing the actual quest is not required to unlock this achievement. You just need to equip a Mysterious Amulet.)
-Punch him: If you choose to punch him, this will end the quest and you will NOT get the amulet nor will you be able to get the quest again. If, for some reason, you do punch him, you can still get the achievement via running it with another character or have someone who has one join your game, drop it, and let you wear it.