So the group escape and regain their gear with little trouble and set off looking for people to rescue. As usual it is hard to make them fight a set battle unless they want to as they have an invisible scout and a very stealthy scout and they do their utmost to avoid anything they possibly can.
In this case they have the old knight as another scout. He still looks like a prisoner and presumably won't raise much notice amongst the defenders they hope. So they see some prisoners they want to rescue, some guards and a patrol heading in. They all retreat leaving the old man to mingle with the prisoners hoping one prisoner looks much like another to the guards and that a rigourous count isn't maintained. The patrol eventually leaves party come back out, patrol repaths to where they are, they retreat again and wait. Eventually the patrol goes away and the PC's blitz the remaining guards and rescue the prisoners who insist on seeking their own way out.
They free the mindflayer and head to the big fight in the sacrificial chamber.
I make a few small reading comprehension errors which was entirely my fault it is explained well enough about the set up for the kelp and the blocking terrain which don't end up mattering much anyway. The sacrifice is initially prevented with a fear effect but the priestess gets back to the altar and saves vs the held for exactly this second fear effect. At the end of the round she performs the sacrifice which upset my players as it seemed she shdn't have any more actions that round. They really don't like things that happen that seem to be out side of normal rules in general. They are still going on about how on earth were they supposed to know they cld close the hole in the moonbridge with skills from the last adventure.
Anyway the creatures were all killed though it was a tough fight.
