Board Thread:Suggestions forum/@comment-27200931-20170426224854/@comment-27200931-20170428013046

Gandalfthegreatestwizard- EpicMithrandir wrote: Hahaha. Very funny. There is no damn lore! Utumno was destroyed completely long before the Third Age. Anything written would be in direct contradiction to lore. "Nonetheless the Valar did not discover all the mighty vaults and caverns hidden with deceit far under the fortresses of Angband and Utumno. Many evil things still lingered there, and others were dispersed and fled into the dark and roamed in the waste places of the world, awaiting a more evil hour; and Sauron they did not find."

It's simple: Utumno was destroyed and some evil servants of Morgoth survived by hiding themselves in caverns and vaults under Utumno, then some time passed and those creatures started to reproduce. Then they returned to Utumno, possibly because of an overpopulation, or maybe because of conflicts between groups or tribes?