The Utumno Remnant are the the remains of the Host of Utumno before its destruction. They currently dwell North of the Iron Mountains as they prepare for their master's call.
|“||The Dwarves became troubled, and they spoke to King Thingol, saying that the Valar had not rooted out utterly the evils of the North, and now the remnant, having long multiplied in the dark, were coming forth once more and roaming far and wide. 'There are fell beasts,' they said, 'in the land east of the mountains, and your ancient kindred that dwell there are flying from the plains to the hills.||”|
–The Silmarillion, J.R.R. Tolkien & Christopher Tolkien
The Utumno Remnant are the remains of the hosts of Morgoth before the Valar destroyed his fortress of Utumno. They dwell in east of the Ered Luin in Eriador, and make their way west. They trouble the dwarves and elves, and cause chaos to anything in their path.
When Utumno was destroyed in the Years of the Trees, the survivors of its host hid deep in the caverns of the citadel. Here they awaited their master's return for thousands of years. When Morgoth finally returned to Middle Earth, the remains of the once great Host of Utumno came out of the shadows as they knew it was time to prepare for the upcoming and imminent war. They marched south and settled east of Ered Luin, in the northern parts of Eriador where they knew they would be safe from the Elves for some time, as they rebuild their strength.
Now as the Siege of Angband is going on, they still prepare for the war to come. Once they are finally ready, and receive the orders from their master, it is not unlikely that they will start their conquest in Eriador and soon after turn their heads west - towards Beleriand where they will help their master in Angband.