Loki: A Bad God's Guide to Ruling the World : Loki, Book 3 (Loki: A Bad God's Guide‪)‬

The third book in the number one best-selling series.
When Heimdall and Hyrrokkin abandon their "children" to go on holiday, Odin sends Baldur – Thor's half-brother and god of making Loki look bad – to babysit.
As Baldur and Thor bond, Loki's attempts to prove he's good start getting a bit desperate. After the drama teacher casts him as the villain in the school play, Loki starts to wonder whether there's any point in being good when everyone seems convinced of his villainy.
That is until he discovers a rather unusual ring among the costumes and props. A ring that looks suspiciously like the cursed ring of Andvari, one that promises Loki the world if only he gives in to his deepest, darkest desires...