The Legend of OTO-HIME

(This story is similar to the legend of Niamh)A young fisherman married Oto-Hime and went to live in her castle below the waves.
They were very happy until he began to miss his family. Oto-Hime gave him a little box that he must carry (and never open) so that he could visit his family. When he reached his home, all of his family and friends had died hundreds of years before. The fisherman was so upset, that he sat down, and forgetting Oto-Hime’s instructions, opened the box. Out came the hundreds of years he’d spent with the goddess. The fisherman withered and died.

Source: Japanese sea deities (The Arcane Archive)