Eikrythmir

Eikrythmir is a magical stag that lives on the Tree of Laeradr in Hotel Valhalla. His horns endlessly spray water that feeds every river in every world.

The Sword of Summer
Samirah al-Abbas mentions Eikrythmir to Magnus Chase at Hotel Valhalla. This happens at dinner time on Magnus' first day at the hotel when Sam is giving him an orientation of the place.