Here is the full list of all 21 Link: The Unleashed Nexus achievements.
Through the "HeavensLeash", and Broke the Alpha Nexus.
Returned from the "HAVEN". But maybe you're still in bind.
Through the shrine and Reach to top of the "HAVEN".
Began the R.E.M.
Through "Ocean of the Silence".
Beat "CS: Observer".
Used "Urgent Ascent" many times.
Beat the 1st Nexus.
Escaped from "The Nexus". But you're still in bind.
Beat 1st Nexus without "Urgent Ascent".
Beat 2nd Nexus without "Urgent Ascent".
Beat 3rd Nexus without "Urgent Ascent".
Beat 4th Nexus without "Urgent Ascent".
Beat 5th Nexus without "Urgent Ascent".
Beat Alpha Nexus without "Urgent Ascent".
Collect 3 Spheres in 1st stage.
Collect 3 Spheres in 2nd stage.
Collect 3 Spheres in 3rd stage.
Collect 3 Spheres in 4th stage.
Collect 3 Spheres in 5th stage.
Collect 3 Spheres in 6th stage.