The Sylleran Rift was a location in the Delta Quadrant. It was outside the part of space containing Tarkan sentries.

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 USS Voyager after the trip to the rift. In the end, Kes decided against the trip. (VOY: "Darkling")

This location was only mentioned in dialogue.