In my homebrew setting, played approximately from 1980-2010, the elves were heavily influenced by Tolkien. Further, the AD&D rules restricted elven clerics to NPCs. So I did me some thinking.

There were High Elves and Sylvan Elves in the Banisis, a huge forested area that stretched East and off the edge of the map, located between a northern cold salt desert, and a Southern prairie.

At that time I had read a piece about Arctic Elves, so I put some in the mountains north of my salt-desert, and while considering why High Elves and Sylvan Elves lived together I noticed that my desert was a bowl surrounded by mountains on three sides, and the forest on the south. The obvious answer was, it used to be a sea.

An inland sea, upon which the elves, at the peak of their civilization, sailed. When the sea receeded, the elves on their islands were isolated. Many migrated South, or North, or West. Those who remained died.

The elves of my setting were not mortal. But like Tolkien's elves, they could be killed, and they eventually tired of the mortal world. As elves got older their attachment to the mortal world weakened and they would begin preparations to move on to the immortal realm.

These elders become elven clerics. Their prayers and rituals give them a luminescence which marks their increasing detachment from the world. Others, both elders weary of the world and younger elves, perhaps lame, suffering great grief, or unwilling to part from one who is leaving, join the elder. Sometimes only a few days are needed, other times decades are required, but when all is ready they begin a pilgrimage Westward. Sylvan elves typically walk or ride mounts down increasingly complex forest trails, but High Elves build swan-ships that sail out onto the Sea-That-Was.

Seldom seen by humans, these vessels are luminous, transluscent, crewed by ghostly elves, and they sail where the sea level used to be. People on the ground see a ghost ship sailing overhead.

Those high elves who are too young to remember the Great Inland Sea, yet old enough to desire to leave the mortal world, sometimes adopt the Sylvan Elf traditions, which makes the Elven Ghost Ships less common. What was once decadal events has become 20 to 50 years apart.