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Warning! This page contains spoilers for The Sword of Summer.
Marvin along with his brother Otis are frequently sacrificed by the Nordic thunder god Thor and reform from the mist of the Ginnungagap a short time afterwards. Magnus first encounters Marvin when Otis brings him, Sam, and Hearth to Thor who is trapped and drowning in a river thanks to a giantess. Initially, Marvin is unimpressed by the reinforcements Otis had brought, but Magnus proves to be useful and manages to free Thor by throwing Jack into the giantess's nasal cavity. To show his gratitude to the einherjar, Thor guts the goats and treats everyone to a nice meal of Otis and Marvin. Later, after the goats had reformed, Otis and Marvin pull Thor's chariot into Geirrod's palace.
Marvin's first appearance in this book is after Magnus, Alex, Blitz, Sam, and Hearth are rescued by Sif from a mob of angry giants and are taken to her and Thor's home in Asgard. Him and Otis are accompanying Thor in his "man cave" when Magnus and his friends show up to discuss their plans about Sam's arranged marriage to the giant Thyrm. Thor suggests for Marvin and Otis to pull the chariot to the wedding hall, saying that he can "see and hear what my goats can see and hear". The two goats comply and pull the chariot into Thyrm's lair. When Thor, Heimdall, and Vidar manage to break into the cavern of Loki where Thyrm intended to exchange his vows with his bride, Thor rushes immediately to check over his hammer, ignoring Otis and Marvin. Marvin clearly doesn't appreciate the gesture, as he states sarcastically, "we're fine, boss. Thanks for asking."
- Marvin's real name, Tanngrisnr, means snarler, since he does so quite often.
- Marvin is annoyed by the fact that he gets sacrificed along with his brother every single time.