Golden Army is an army of mechanical golden soldiers build with the sole purpose of slaughtering humans. It was commissioned by King Balor during the war against the humanity. The Army can be controlled by the members of the royal family with a golden crown, consisting of three pieces. ( in the movie )
It is said that once humans and magical creatures lived in harmony. But the humanity was born with a hole in their souls, a whole that nothing could fill. And so the war began. Elves and their magical cousins were desperate until one day a Goblin blacksmith came to king Balor with a proposition. He offered him his services in building an invincible army of mechanical soldiers that would make the elves victorious over humans. King's son - Nuada - urged his father to agree, and so he did. And so the blacksmith built an army. To control it he also forged a golden crown for the king.
The next time humans marched against magical armies they faced a foe they could never hope to defeat - the Golden Army. Mechanical warriors butchered humans quickly and elves emerged victorious. But there was a price to be paid. The Golden army did not distinguish between friend or foe, they had no mercy on the innocent human women and children.
Horrified by his actions, king Balor decided to stop the madness of this war and signed a peace pact with the humanity giving them control over cities while retaining control over forests to his people. The golden crown was divided into three fragments and separated: one piece was given to humans and two to elves (king Balor himself and his daughter Nuala).
Prince Nuada, however, didn't trust humans and believed they would betray elves, so he left into exile, swearing to return in his people's hour of need.
The Golden army was sealed off underground, lying dormant.