In Act II, as the player travels through the Sewers, they'll come to a room with toxic barrels and a few noxious Walkers, as well as a Floater. There will be a ladder in the middle which they should climb up and follow the path around to the Purple Skull, which rests in front of a shrine. The shrine contains a picture of a woman and her daughter. It should be noted that the player passes through the location a few times during the campaign.
In Act II while in the city of Moresby, there is a side quest available on the east side of the map called "Uninvited Guests". Once upstairs inside the apartment, the player should head into the kitchen and place the Purple Skull on top of the stove.
Once the player completes the quest and leaves the house they can not reenter the building. It's recommended to make sure that they have the Purple Skull before they complete the quest. However, it's possible to re-enter the apartment if they avoid talking to Alicia Jacobs in order completing the quest. As of Patch 1.3, the player can return to the altar even after having completed the side quest.
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