The Thorn was a highly unstable area of space. (Star Trek: Mirror Wars)
The Thorn was where the first ship of the Mirror War was lost, the USS Enterprise (NCC-1701-O. Starfleet learned that an unstable area of space made it possible for ships to cross between universes, and planned to enter the mirror universe through the Thorn.
Starfleet opened a portal to the mirror universe in the Thorn and primary universe ships started to cross over, taking the war to the mirror universe. (Star Trek: Mirror Wars: "Alternate Invasion, Part 2")