Umbral

The language of shadows

"Umbral is so in tune with darkness, that it can wreathe a speaker in shadows. It is a shame that no thief or burglar knows it, because that would be fun to watch"

- Makaria Ambition, Archmage of Howarde's Adventuring Guild

Umbral is the language of darkness, words of shadow, sounds of eclipse. Merely knowing the language is enough for someone to become more difficult to see in the dark. Umbral is innate to the Shadowfell, and as such is one of the oldest languages along with Primordial and Sylvan. While the Shadowfell has many similarities to its counterpart the Feywilde, Umbral shares few words, or even letters with its counterpart Sylvan. Despite Shadowfell's ruler's best effort, mortals were more drawn to the beauty of life than the harshness of death. Umbral was replaced by Sylvan, which was replaced by Common by even drow and other dark natives to Aruna. The only people who know Umbral now are Shadowfell natives, scholars, and dark fey such as hags.
Umbral is a harsh language with crisp consonants an little vowels. People used to speaking Elvish or Sylvan find it near impossible to figure out Umbral because of its difference from the flowing languages of life. Speakers of Nekrul, however, have a very easy time figuring out Umbral, because undead are very close to the Shadowfell and innately the languages were made similarly. Other languages that were based off Umbral were Abyssal and Infernal. Also runes that have dark or necromancy based spells have similar pronunciations as words in Umbral. 
The Night Wolf Cloak that is worn by the Onibaba of Soraxis has an inscription in Umbral. It reads: The immortality of darkness, the speed of shadow, the beauty of eclipse, all given to the Onibaba. The horror of darkness, the avoidance of shadow, the hate of eclipse, all beared by the Onibaba. No other primordeal artifact is known to have an inscription in Umbral.

Mechanics

Once per day, you can have an additional +1 on a stealth check.

Native Speakers
Shodow Fey, Shadowfell inhabitants, shadow demons