10 Doctor Who Characters Who Can Never Die
4. The Trickster
RTD's family-friendly spinoff The Sarah Jane Adventures introduced a handful of immortal entities to Doctor Who lore, from the Dark Empire's tyrannical leader, Horath, to Erasmus Darkening (quite possibly the best villain name ever), an alien scientist who took the lives of others in order to prolong his own.
But with Horath being light on screen time and with Darkening being defeated, The Sarah Jane Adventures' biggest and best immortal is easily the Trickster, a pale-faced, devilish creature whose idea of a perfect day out involves wreaking havoc on a universal scale.
In The Wedding of Sarah Jane Smith, Rani describes this meddlesome monster as an "all-powerful immortal", and it's mentioned that the Trickster is part of a wider group, one whose goal is to alter reality and cause chaos: the Pantheon of Discord. The Tenth Doctor states that he's known of the Pantheon since he was a kid, and considering that this dashing Time Lord is pretty darn old, it's clear that the Trickster and his pals have been around for a good long while.
In the lockdown webcast Farewell, Sarah Jane, it's mentioned that the Trickster is shrunk to the size of a doll, and locked in a treasure chest for the next thousand years. And that's really all you can do with this guy - you can contain him, sure, but you can never properly land that fatal blow.