The Sightless Eye is an artifact once possessed by the nephalem Philios. He used it to maintain contact with the angel Lycander, after she fled Sanctuary in light of Lilith's purge. When her fellow angels learned that she was communicating with someone outside Heaven, she ended the romance and insisted that Philios hide the Eye so that Heaven would not learn of Sanctuary's existence.
Philios hid the Eye on Skovos, where it was recovered by his twin daughters, where they made their home on the island. Though they did not use the Eye to communicate with Heaven, they found that they were able to see the future through its mirrored surface. So it was that Askari culture developed, with the Sightless Eye at the heart of their society. Even today, it is held as one of the most sacred objects in Askari culture.
That being said, the Eye no longer resides on Skovos. A group of Amazon dissidents called the Sisterhood of the Sightless Eye stole the artifact and fled to Eastgate Keep in Khanduras. Its current status is unknown, though Deckard Cain noted that it falling into the hands of Andariel was one such possibility.