Nidavellir (meaning "the dark fields"; pronounced "nih-DA-vel-EAR") is one of the Nine Worlds and is the home of the Dwarfs. Being all underground, there is no sunlight and is instead lit by glowing moss.
Magnus Chase and Blitzen are here sent by Freya to get her new earrings. They then run into Samirah al-Abbas and Hearthstone. After the crafting contest they are chased out of the world and escape by jumping into a river.