Midnight entity

When the Crusader Tour shuttle breaks down on Midnight, the mechanic notices a strange thing which he claims to see it running. :This may mean that the creature is humanoid.

During the Crusader Tour to the Sapphire Waterfall, a strange monster attacked the tour bus. The monster knocked on the Shuttle and then possessed Sky Silvestry and the Doctor, before the Hostess threw herself and Sky off the shuttle to save the Doctor. (DW: Midnight)

The entity may have survived the death of Sky but it may still be lurking on the planet of Midnight.

Behind the Scenes
This is the first time on Doctor Who where an enemy has never been revealed. The name of the creature is not revealed in the episode, and "Midnight entity" is just its unofficial name.