Just read the summery online, and I see we are raping this story. In the movie the cauldron first is under ownership of three witches and Hen Wen already had been under protection of fairies. However The Horned King's army takes the cauldron and puts everyone, but Gurgi, prisoner. They escape, Gurgi sacrifices himself to stop the skeleton army, Horned King is swallowed because the cauldron is out of control. Gurgi revived by witches in exchange for the cauldron, happy end.
I think our story will go like this: everyone imprisoned trying to save Hen Wen. Cauldron already under possession of Horned King. Heroes fight Horned King, save Hen Wen, Horned King is sucked into the cauldron, magical powers of the cauldron release the Sign inside of Hen Wen. Maybe Gurgi still can sacrifice himself, when he runs away from Eilonwy and Fflewddur who were said to stay behind. Those two also follow and watch the fight.