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 of the large, open area which they should climb up and follow the path around to the Purple Skull, which rests in the corner 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. The closest sewer entrance to where the purple skull is found is directly in front (at the Northeast corner) of the Police Station. Or, when you are at the church you need to go down the road, where if you turn right there will be a bridge with an overturned truck. if you go under the bridge there will also be another entrance to the sewers, where the ladder will be in front of you, along with two toxic walkers and a floater, but this is a bit longer route.
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.
In game versions prior to Patch 1.3, after the player completes the quest and leaves the house, reentry is not possible, so the Purple Skull must be obtained before leaving. It was possible to re-enter the apartment if the player had not talked to Alicia Jacobs, thereby completing the quest.
As of Patch 1.3, the player can return to Alicia's house - and the altar - at any time, even after completing the quest. However, there is an infrequent bug which may temporarily prevent the entrance from being accessible. Leaving the Moresby map, even just to enter another Abandoned House, and then returning should make the door accessible once more.