Deep Giant and Snow Daughter, having sworn the Oath of the Truehearts, go to Arcadia in order to prompt the reincarnation of the Mountain King.
[Because of the split into two factions after the Mountain King's death, there has been no oathbound troll couples to wait for the king.]
The Sidhe, knowing the power of the Mountain King, had no wish to see him reincarnated, and sought to block Deep Giant and Snow Daughter from getting to the Gate of Ivory. However Lady Eiluned helped the two Trolls gain access to the Gate, keeping the Sidhe at bay while they waited in the antechamber.
[The previous bit needs to be revised. The Sidhe were not in Arcadia at the time, all its fae occupants having been forced out by the Fomori. The Sidhe may have tried to block the Trolls from reaching Arcadia, but Lady Eiluned's great service was to protect the Trolls from the Fomori, allowing them to safely enter and leave Arcadia.]
When the Mountain King reincarnated, Deep Giant gave himself over to the Sidhe as a hostage for the good behaviour of the Mountain King.
See also: the Troll Revolt.