10 Darkest Doctor Who Moments

10. The Boneless' Murals (Flatline)

The Boneless were belligerent beings from a two-dimensional universe, who caused the Twelfth Doctor all sorts of problems by shrinking the TARDIS exterior while he was inside it.

They eventually converted into surreal-looking 3D monsters that could've come straight out of a Salvador Dali painting. Now there's a killer episode concept.

Terrifying though they were emerging from the ground and giving chase in a suitably dark tunnel, the Boneless were at their most sinister in 2D. The pre-credits sequence shows a frightened man on the phone to the police, saying "I can't speak up, they might hear me... they are everywhere", before being snatched from underneath, and ending up on the wall as a screaming mural.

Later, PC Forrest is also muralised as she searches a missing person's room. The Boneless slither towards her, then brutally pull her into the floor. Barely a second goes by when Clara and Rigsy burst into the room after hearing PC Forrest's screams, only to discover her nervous system graphically displayed on the wall like a twisted art piece.

The episode's writer Jamie Mathieson believed that Doctor Who should be "utterly terrifying and filled with body horror". Stringing out people's innards was certainly one way to make that statement, demonstrating that the Boneless were savage creatures who could not be bargained with.