The following skull can be picked up in Forge mode on the map Heretic
From the top middle of the map, go into forge mode, fly over to the window of the blue base and you should be able to see this skull sitting on a ledge above the front facing window.
Exit forge mode and make sure you pick up the skull.
It can be seen in the video below, and I have included a timestamp of its exact location:
] - Skull 5 - Heretic