While the other Sidhe were fighting at the Battle of the Riven Mountain, Lord Balor and his House took control of Arcadia.
When the Sidhe returned from the Battle they were unable to enter Arcadia, for Lord Balor had ordered the Gate closed and locked. Lord Gwydion was unwilling to use violence, for none wanted Sidhe to fight Sidhe.
The situation was resolved when Lord Leanhaun turned up, along with the members of his House. Ignoring the taboo against violence between Sidhe, he and his followers stormed the Gate, and then set about killing the members of House Balor. Lord Leanhaun went after Lord Balor and killed him, though Lord Balor made no attempt to fight but rather parlayed.
Thus Arcadia was liberated, and Lady Fiona gains an excuse to destroy House Leanhaun.