Possible Clues:

• Villain in 2011's 'Thor'
• Regular in the ''Thor'' films
• Shape-shifting god
• Asgardian rogue
• Shape-shifter of Norse mythology
• Parent of Odin's horse
• Thor's mischievous adoptive brother
• Mischievous shape-shifting god in Norse mythology
• Norse god found in 56-Across
• Norse god of discord and mischief