adj. unseen, hidden
adv. hiddenly, in an unseen manner
n. hiddenness, the quality of being unseen; something that is unseen
v. to hide, vanish, disappear, make or become unseen

Qethsegol vahrukiv sod do Siglif fin Vonun wo aam lot Keizaal ni ol kendov ko vu nuz ol sah ko suvulaan.
This stone commemorates the deeds of Siglif the Unseen, who served great Skyrim not as a warrior in the dawn, but as a phantom in the twilight.
(Unused Word Wall for Sah, Phantom Form)

From an unattested word nun ‘seen’ (possibly ‘to see or spot’) with the prefix vo- ‘un-, opposite of’.