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The river Isen occurs in J. R. R. Tolkien's fictional universe of Middle-earth.
The Isen (or Angren in Sindarin) began in the southern Misty Mountains, first flowing south through the fortress of Isengard (or Angrenost) to the White Mountains, where it abrubtly bent west and flowed to the sea of Belegaer.
At a distance of about 150 Númenórean miles west of the Gap of Rohan, the Isen was joined by its tributary, the river Adorn.
The Isen and Adorn formed the boundary of the Kingdom of Rohan, but the triangle of land between Isen, Adorn, and the White Mountains was a contested land, claimed by the Rohirrim as well as the Dunlendings.
The Isen formed a natural boundary in the Gap of Rohan, and was only crossable at Isengard or at the Fords of Isen, where the Rohirrim fought a number of great battles against the Dunlendings and Saruman's Orcs in the late Third Age.
Saruman diverted the Isen away from Isengard as he turned the fortress into a war machine, and it was dammed at the northern wall. When the Ents attacked Isengard, they broke the dam and restored the original flow of the river, temporarily drowning all of Isengard.
ISEN also stands for the Internet Search Environment Number. ISEN is a standard identifier for database URLs and their corresponding database server subnet Internet Protocol addresses. ISEN is a database of databases.
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