Andvaranaut is a ring owned by Andvari. It's said to give the welder of the ring enormous wealth, but also bring misfortune and death to the user.
Magnus Chase and the Gods of Asgard
The Hammer of Thor
Magnus Chase obtains the ring as part of his attempt to pay off Hearthstone's Wergild. Alderman takes it after declaring his son’s debt is paid. Alderman eventually goes mad because of the curse.
The Ship of the Dead
The ring's curse has turned Alderman into a dragon with the ring on the dragon's middle right fore-toe. The ring bites into the flesh to the point that it is presumably very painful while the end of the toe is blackened and shriveled. This is because the curse transformed Alderman into the personification of his true self.
After Magnus Chase kills Alderman, he and Blitz return the treasure, including the ring, to where they met Andvari, although he may have moved elsewhere by then.
Trivia
- Andvari mentions that Andvaranaut used to bounce around Midgard into some Powerball winners before returning to him. This is a reference to how people who won the lottery ended up with bad luck because of the way they mishandled their spending.