Birthmark

All of the player characters are born with a birthmark. The emblem of the Everwatcher, a crescent with a dot in the middle. The birthmark is different for each character and can show up in any location, and size, and any style. Examples of styles may be discolored skin, discolored hair, scaring, scale patterns, etc.

This mark was born with you and has been a part of your life for as long as you can remember. However it may or may not have ever effected your life. Those aflicted by this mark are randomly summoned by the Everwatcher to complete their bidding.

Meta Mechanical

This mechanic, while slightly awkward, is a core mechanic of the entire campaign to allow for players to appear and vanish from the active party without panic when their player characters become unavailable.