Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka OVA

Is It Wrong to Try to Pick Up Girls in a Dungeon?: Is It Wrong to Expect a Hot Spring in a Dungeon?

On their way back from the 18th floor, Hestia, Bell, and the others accidentally uncover a hidden, mystical hot spring, and they decide to stop by to refresh themselves until creatures lurking in the dark appear to attack them.