Baldur the Beautiful, (PROSE: A Hundred Words from a Civil War) or Balder, was an Aesir god in Norse mythology.
In 141, the Third Doctor and Jo Grant met the human Odin and his sons Thor and Balder who inspired the Norse legends. (PROSE: The Spear of Destiny)
Loki was the "blood brother" of Odin until he killed Baldur (PROSE: Where Angels Fear) with mistletoe, which was his only weakness. (PROSE: Timewyrm: Revelation)
In 1070, the Eighth Doctor told Izzy Sinclair that the Norse worshiped gods that included Frey, Thor, and Baldur. He also said they were more "rough and tumble" when compared to their comic book personas. (PROSE: Illumination)
A Remake of Baldur in the City of the Saved caused the "invulnerable-Baldur cock-up". (PROSE: Of the City of the Saved...) A remake of Balder was killed with a branch of mistletoe in custom-killing of deities. (PROSE: A Hundred Words from a Civil War