Mandrake (The Widow's Assassin)
From Tardis Wiki, the free Doctor Who reference
Mandrake the Lizard King was an imaginary enemy that the First Doctor created as a child to combat his loneliness. When Crozier used his machine on the Sixth Doctor, it entered the machine and became a mind parasite. When Crozier went to place Lord Kiv in Peri Brown, it placed itself in her mind. When the Doctor came back for Peri, he tried to convince the Doctor to transfer what he thought was the Lord Kiv entity into Princess Dirani. When the Doctor transferred his memory into Peri, he wanted to know why no-one had heard of him; this was because he was a creation from the Doctor's imagination when he was lonely. (AUDIO: The Widow's Assassin)
