Skull

The skull was the framework of the head in most animals, including humanoids.

On Earth, and later human colonies, a bare skull was a commonly understood symbol for death. Death took this form as an emoji: 💀. (TV: Smile)

Female Rills found a large skull sexually attractive. (PROSE: Galaxy Four)

The skulls of the headless monks were kept in the Seventh Transept. They were carnivorous and ate anything that came in, from rats to intruders. (TV: The Wedding of River Song)

Faction Paradox wore the skulls of dead Time Lords from another universe. (PROSE: Alien Bodies)

Victorian showmen used to draw the crowds by taking the skull of a cat, glueing it to a fish and calling it a mermaid. The Ninth Doctor compared the Slitheen's Space Pig with this trick while explaining the Space Pig's true nature to Toshiko Sato. (TV: Aliens of London)

The Marshal of Atrios communicated with the Shadow through a skull-shaped device located behind a mirror. (TV: The Armageddon Factor) Broke of the Cult of the Dark Flame forced Victor Farrison to search for the skull of Vilus Krull's original body so its energies could be used to summon the Dark Flame. (AUDIO: The Dark Flame)

Inside his confession dial, the Twelfth Doctor was killed and reborn many times by the Veil, but every time, he discovered a skull that had once been his own. (TV: Heaven Sent)

Behind the Scenes
The skulls in Heaven Sent were actually modeled after Peter Capaldi's own skull.