plot bases. Other characters based on Heimdall, Frigga, and Odin also appear. Thor also appears in the 2012 crossover film The Avengers, with Loki also returning as the villain, and returned in the 2013 sequel Thor: The Dark World and returned in the 2017 film Thor: Ragnarok. Thor also appeared in the 2015 crossover film Avengers: Age of Ultron, the 2018 crossover film Avengers: Infinity War and the 2019 crossover film Avengers: Endgame. These films are set in the Marvel Cinematic Universe which crosses over the film properties owned by Marvel Studios (i.e. Iron Man, Captain America, Thor, Hulk etc.). Ingmar Bergman's The Virgin Spring (1960) provides a meditation on the persistence of the old religion in medieval Christian Sweden when the dark energy of Odin is unleashed by a character's secret worship of the god. The DreamWorks animated films How to Train Your Dragon (2010) and How to Train Your Dragon 2 (2014) feature Vikings who make a few references to the Norse gods Odin and Thor and show passages written in runes. Video games The Ragnarok Online universe contains many references to Norse mythology – in fact, almost everything in the game is based on Norse mythology. The plot of Ash of Gods: Redemption is significantly based on Norse mythology, including names of the Runes and Gods used as the names of states and cities. In Ace Combat Zero: The Belkan War, the player is part of a two-man mercenary squadron called Galm Team: a reference to Garmr, a Norse hellhound. Tomb Raider: Underworld uses the Norse mythology keystones, names and artifacts as its main plot basis. Protagonist Lara Croft, in the course of her search for her mother, visits the Neiflheim and other mythical places. Thor's hammer Mjollnir is used as her weapon. In StarCraft, four of the fifteen Zerg Broods are named Fenris, Garm, Jormungand, and Surtur, and there is also an overlord hero unit called Yggdrasil. In addition, there is a Terran unit called a Valkyrie; similarly, in Starcraft 2, there are several Terran units that are named based on Norse references. The role-playing game Dungeons and Dragons has included Norse gods as optional elements since the publication of its Deities and Demigods sourcebook. The tri-Ace role-playing game Valkyrie Profile: Lenneth is based on Norse mythology, though it does deviate at some points. The main character is a valkyrie named Lenneth, whom many have thought