Craglorn is an Adventure Zone. This area is designed for a group of 4-12 Player Groups of Veteran Rank 10 (VR 10). This area is designed to bring them to the new cap of VR 12. It is accessible as early as VR 1, but it is not recommended.
This area is accessible by talking with Star-Gazer Herald. Once you find one of the Star-Gazer Heralds, you will receive the quest: The Star-Gazers. The Heralds are found in 3 locations across Tamriel.