Thugheart was the founder and the first Chief of the Outcast Tribe (which was previously called the Treacherous Tribe). He was the eldest son of Grimbeard, and the paternal grandfather of Alvin the Treacherous.
Thugheart was the eldest son of Grimbeard the Ghastly, he was a competitive Viking who made his father proud. When Hiccup Horrendous Haddock II became Grimbeard's favorite son, the jealous Thugheart plotted to overthrow his father and younger brother. He conspired with the Uglithug King of the Mainland. He convinced Grimbeard that Hiccup the Second was a traitor, causing Grimbeard to kill Hiccup. When Grimbeard soon realized his mistake, and who the real traitor was, he fought and defeated Thugheart and the King of the Mainland. Thugheart was banished to the Outcast Lands, where he rallied some exiles into a tribe: The Outcast Tribe. He married Sychofanta, and had one son. His grandson, Alvin became chief. Thugheart died of old age later.
Thugheart was described as being the spitting image of his father, Grimbeard the Ghastly.