"Well, you need dragons to conquer other dragons"
"Or maybe you need dragons to conquer people, to control those who follow you and to get rid of those who won’t."
―Hiccup Haddock III & Drago Bludvist
This Army was led by the self-proclaimed "Dragon God", Drago Bludvist
"But even as a boy, left with nothing, I vowed to rise above the fear of dragons, and liberate the people of this world."
When he was boy and lost his family, Drago vowed to rise above dragons. Thus he set himself to conquer the world by using the dragons. During his conquests, he then found a Bewilderbeast hatchling and raised it with cruelty. He then assembled the army composed of human dragon trappers and captured dragons.
Drago then went to a gathering of Chiefs on the Isle of Berk. There he demanded the chiefs to be commanded and follow him in order to end their war with the dragons. But the chiefs laughed at this and send him away. Drago then left and sent his armored dragons to burn the hall down. All but Stoick were killed.
For the next 20 years, Drago built up his army; including tribes from faraway lands such as Slavic, Sami, Inuit and Asiatic. He had many traps, weapons and armor made for his enslaved dragons. He used trappers such Eret to build up his army. He also had one of his men buy dragon from the Dragon Hunters' Chieftain Viggo Grimborn's Dragon Auction. Nothing else from Drago's human army remains until the end of the sequel, possibly returned to where they'd came.
This is the first tribe in the franchise that has not a specific name.
Drago's army could be connected to the Dragon Hunters due to the fact that the "special guest" have Drago's insignia.