This ruined fortress is located at the south-eastern corner of Nain Enidh, just where the sheltering ridges bends south for the marshes of Harloeg. These days orcs make camp in the ruins where they have formed an alliance with the Dourhands of Thandobel.
Somewhere within Ost Cyrn there is a door that leads into the Iorvinas dungeon, at the Orc side of the dungeon.
The following deeds can be obtained by visiting this location:
-  A Plague Upon Us
-  Iron-jaws
-  Man-slayers
-  Master of the Lash
-  Instance: A Champion's Courage - Champion
The following creatures are found within this area:
- Ost Cyrn once stood vigil against the Hillmen of Rhudaur and the Angmarim of the North. Now Orcs use the ruined fortress to lay siege to the barren Lone-lands.
|Settlements, Areas and Landmarks of the Lone-lands|
|Settlements:||The Forsaken Inn • Ost Guruth|
|Areas:||Agamaur • Annunlos • Garth Agarwen • Haragmar • Harloeg • Minas Eriol • Nain Enidh • Talath Gaun • The Weather Hills|
|Landmarks:||Amon Ros • Barad Dhorn • Barad Iachiant • Bleakrift • Candaith's Encampment • The Circle of Blood • Dol Vaeg • Echad Sûl • Eglain Camp • Garth Agarwen Gate • Glumhallow • Gondrinn • Iorvinas • Ivar's Crypt • The Last Bridge • The Lornspan • Mithrenost • Naerost • Nan Dhelu • Nindor • Ost Cyrn • Ost Haer • Ost Laden • The Outer Gates • Skirmish Camp • Spring of the Red Maid • Thandobel • Tornstones • Weatherfoot • Weathertop|