Soon's gate is a gate made by Soon, the head of the Sapphire Guard when he and his adventuring party went and sealed all of the gates to the prison. It was actually a sapphire adorning the Lord of Azure City's throne.
Xykon sought to get hold of it; O-chul attempted to destroy it, but Xykon managed to paralyze him. Then Soon's spirit summoned the ghosts of the dead paladins and a fight erupted. Xykon and Redcloak were nearly defeated, when Miko Miyazaki, released by the Twelve Gods from her prison, entered the throne room. Unknowing that the Gate was now safe and the enemies soon to be defeated, ran to destroy the Gate so that it would not fall in their hands.
The result was a huge explosion that destroyed a part of the City. This break allowed Xykon and Redcloak to get away since the ghost's were now no longer bound to the world.