After the nine Sidhe arrived in Arcadia, they each went their separate ways. Lord Dougal remained in Arcadia and built the Gate of Horn; Lady Eiluned travelled to the high reaches of the Splinter Mountains and spoke with the wind; and Lord Balor travelled deep into the dark places of the Dreaming. He sought those places where the trees grow tall and thick, where the soil is thick and moist, and where the sun was as bright as the moon. He walked through summer and autumn and into winter forests which never let their leaves fall.
There, in a place which wanted no name, Lord Balor built a house for himself beneath the slate sky. He struck down the mightiest of firs and fashioned his dwelling from them, and took thick moss from the earth to seal it from the north wind. He moved rocks and water until he was pleased with his new home.
Lord Balor stayed there for a timeless time, but it did not stay as he wished. The wooden walls buckled and cracked, the moss shriveled and died, and water slunk in through the floor. Always he had to fix some problem, and once fixed a new one would appear. Yet still he remained, for he thought his home would eventually be perfect.
Then one day Lord Balor noticed that nothing remained to be done, nothing needed fixing, and joyful he walked out into the forest. He wandered without a care, and barely noticed the land around him change. The trees became closer and closer, their trunks first touching and then twining around each other, branches enmeshed, their leaves blocking out any trace of the sun. The ground grew hard and rough, and the only sound was his own footsteps. Only when the trees stopped being trees, and were instead walls of stone, and the only light came from a distant flame, did Lord Balor notice that he had wandered into another place. Then he looked round, and saw that all about him was solid rock, save ahead towards the light.
So he walked forward and came to the great halls of Undermountain. Here dwelt the Mountain King, who sat on a throne carved from the root of the mountain, and all his followers were about him, great Trolls with eyes large and black. Here too was the Dragon Serpentine. The forest was her domain, and she had come to Undermountain to receive the judgment of the Mountain King on the invasion of her lands by Lord Balor.
When he heard that he was to be tried for the crime of invading and corrupting the domain of a Dragon, Lord Balor lept forward, exclaiming that had he known there was a Dragon in the region he would have killed it before building his house. So saying he drew his blade and would have attacked Serpentine had not a score of Trolls intervened, holding him back. So strong were the Trolls that Lord Balor was afeared, and agreed to abide by the justice of the Mountain King.
Then Serpentine listed the crimes of the Sidhe lord, that he had entered her domain without her permission, and had corrupted it by building his house and shaping the land to his will. Lord Balor defended himself by saying that he did not know the forest was guarded by Serpentine, and all he had wished was to build a home for himself.
For a long time the Mountain King was silent, and did nothing but stare into the bright emerald that hung about his neck. At last he spoke, and he passed judgment that Lord Balor must leave the forest and never return.
This Lord Balor swore to do, for he had no wish to go against the Mountain King's wishes while within Undermountain. But when he returned to his home, he no longer feared the Trolls, and resolved never to leave his home. He prepared himself for battle, clad himself in bright armour, put on his helmet of polished bronze, and drew the long blade that hung always by his side. So armed he sought out Serpentine in her lair. He did not have long to search, for Dán worked to bring the two together; Lord Balor came to a clearing in the forest, and there waiting for him, scales glinting in the pale light of the sun, was the Dragon.
None were the words between them; they each lept to the fight without pause. Ever she clutched him, and ever he hewed her, and the sky lit up with sparks and fiery breath. Both were sorely wounded, but in the end Serpentine was vanquished. As Lord Balor pulled his now dull sword from the breast of the slain Dragon, he was beset by the terrors of his oathbreaking, and forced to flee. For many days and nights he ran, pursued, until he came once more to Undermountain.
Desperate, he tried to find some way into the great halls, but the rock face was hard and closed. Crying out he begged for the Mountain King to hear him, to remove the terrible suffering of an Oathbreaker, but he was answered only with silence. None came out to meet him; the Mountain King was firm in his judgment. For so long as Lord Balor waited for admission to the halls, so long was he denied, until at least he fled that place, and returned to Arcadia.
Once there, he told his fellow Sidhe the story, that the Mountain King had passed judgment on a lord of the Sidhe, denying him the right to make home and kingdom, and to fight Dragons. The Sidhe were shocked at this, and resolved at once to go to war. Lord Aelil alone tried to stay their hands, for he knew much of the horror that would come of such a conflict. Yet Lord Aelil was but one voice, and the fury of the Sidhe drove them into battle, so that they would never be ruled again.