The story
The first sense is from Shadow Realm, an invented place by 4Kids Entertainment for their English dub of the Japanese anime series Yu-Gi-Oh! Duel Monsters (2000–2004) as an ostensible place of suffering where people could be sent as a euphemism for death. It does not exist in the original Japanese anime. Apparently based on Japanese 闇(やみ)の世界(せかい) (Yami no Sekai, “World of Darkness”), an actual location in the series which is also called the Shadow Realm in the English dub.
Etymology from Wiktionary, CC BY-SA 4.0.