When you enter the Mausoleum Magnum found at [-7,-3] you will meet someone who gives you the quest, continue up in the room and you will find the door to the Tombs, drop down and kill all the skellies, then open the coffin, grab the black amulet, which will cause more skellies kill them, rinse and repeat 10 more times

once you finish return to the start and talk to him again, and he will give you the key.

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