Complex sentence: Suddenly, the celebration is interrupted when the Horned King's soldiers, having finally reached the marshes themselves, seize the cauldron and everyone but Gurgi, and take their prisoners back to the castle.
Split into simpler sentences: Suddenly, the celebration is interrupted when the Horned King's soldiers have finally reached the marshes themselves. They seize the cauldron and everyone but Gurgi, and take their prisoners back to the castle.