Umbral (CC)
Umbral is a shadow elf language granting +1 to Stealth checks once daily.
Umbral is the language of the shadow elves, a corrupted dialect of Elvish, and spoken by most creatures of shadow. Those who speak it can gain a +1 to one Dexterity (Stealth) check 1/day.
Source and licence
From Creature Codex.
Licence: Open Game Content (OGL 1.0a).