Fairy folk - in traditional mythology, fairies and the like aren’t sweet little pixies, they’re very alien fellows who abide by strict-but-seemingly-arbitrary rules. To differentiate them from modern conceptions, the older term ‘fae’ is often used for traditional fairies.

The power of words and contracts are common themes with the fae, and often in mythology knowing their ‘true name’ gives power over them, so they’re reluctant to give away that information.