The Sylleran Rift was an astronomical feature in the Delta Quadrant.
In 2373, Zahir asked Kes to accompany him in exploring the Sylleran Rift. As Zahir's ship had high warp capabilities, they could have rendezvoused with Voyager after the trip to the rift; in the end, Kes decided against the trip. (VOY: "Darkling")