Ectoplasmic Residue is a zombie-related gameplay achievement/trophy featured in Buried for Call of Duty: Black Ops II. This achievement/trophy requires the player to acquire a perk without purchasing it from a Perk-a-Cola machine. In order to acquire a free perk the player needs to kill a specific number of the Mistress' (found in the manor). The last one will drop a Random Perk Bottle. It may also be possible to acquire the free perk through the Easter Egg as it awards all perks.
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