Semue Iera

From LSWiki

If you enter from Tainaron, you start at -19, 2, 0.

Semue Iera is at (3,3,3) River Tethys. You can also do "go 5e, 6n, u, 3n, 4e, u, 4e, 4s, u, 4e, 4s, 5e" from the Hoop of Fire in Tethys. (flight may be required. return trip: go 5w, 4n, 4w, d, 4n, 4w, d, 4w, 3s, d, 6s, 5w )

Make sure you can gain knowledge of the gate, whether via lens of insight or otherwise. Mounts, unless they have the gate identified, cannot enter, so you will have to dismount.

You can read (not technically, looking at them works) the runes (do so twice) that are on the walls of the room, and if you have enough rune lore, you will identify Semue Iera automatically.