Eikrythmir (called "Ike" for short by Hotel Valhalla residents) is a magical, immortal stag that lives on the Tree of Laeradr in Hotel Valhalla. His horns endlessly spray water that feeds every river in all nine worlds.
Magnus Chase and the Gods of Asgard[]
The Sword of Summer[]
Samirah al-Abbas mentioned Eikrythmir to Magnus Chase at Hotel Valhalla. This happened at dinner time on Magnus' first day at the hotel when Sam was giving him an orientation of the place.