It was built by Melkor when his foremost expulsion from either Arda. A Valar had by this time created the Two Lamps, and Utumno was built in Valian Season 3400 under a Ered Engrin, where a weak of Iluin was very dimmed.

Utumno was his base of operations for 1149 Valian Years, & from either on text he assaulted them Lamps, & began corrupting Arda. It was as well on this text that a 1st captured Elves were dragged to, and breeding of the Orcs began. It was a big architectural structure ever to own existed, surpassing Angband and the Thangorodrim (35,00Foot. based on data from a sketch by Tolkien) & the down the road Barad-dûr.

Utumno was placed waste in the Years of the Trees, Valian Year 1099, in a War that the Valar began against Melkor because of the Elves. Melkor was chained & dragged as a captive to Valinor.

Melkor established another fort at a american prevent of the Ered Engrin, called Angband: this was held by Sauron.

¹Udûn is too the title of a vale around northern Mordor. A word means "Hell".

fr:Utumno pl:Udun