Whilst we are lead to believe that this Eye is given by the Mothers (Fates), what the Eye actually is and does is up for debate.
It seems implied that it can check possible futures but to what extent and how far is unknown - but it seems oddly similar to how the Mothers see futures. One way this might be possible is if the Gate is like the Ultimate Gate in Lovecraft mythos, which one can access the Multiverse through and see all possible timelines. Perhaps the Outer Gates have a similar quality...although that might imply that the Outer Gates are actually a part of the Mothers.
We also are told that it is a tool to check for Nemesis. It doesn't use the Sight however, although it is implied Rashid has used the Sight on Nemesis so it seems these are different methods. Considering it is made from the same "crystal" as the Outer Gates it appears it might share qualities (such as checking for Nemesis among other things...perhaps it also acts as a filter of sorts so that the viewer doesn't go mad).
In mythology there are a number of ancient eyes with mystical properties. The Eye of Horus that was cut out of the god Horus during his battle with his evil uncle Set, was healed by Thoth and was associated with the Moon. Odin cut out his own eye for wisdom and sacrificed it to the Well of Fate. Considering Thoth and Odin are considered to be equivalent (via Hermes/Mercury) perhaps in the Dresden Files that is how Vadderung lost his eye. Even more interestingly is the fact that the eye of the Fomorian King Balor was so powerful that it's evil gaze could kill those it hit (one of the origins for the phrase "The Evil Eye) and after being cut out was contained in crystal. Also, we have had reference to the Evil Eye in Blood Rites - the Malacchio entropy curse that Lord Raith uses (possibly sponsored by HWWBh).
In any case, the complicated links between Vadderung, Rashid, the Mothers, the Fomor and the Outsiders make for fun theories.
Does anyone have any other links or bits of trvia/mythology or clues they have picked up?