I am really keen to see how this turned out!
I am running my PC's and they beat the Alhoon too. For CR 19 it really is very weak with a very poor selection of spells. They trapped the thing at the entrance to the tomb (they had closed the doors so they couldn't be pursued by cultists). The priest tried to Heal it (which acts as Harm) with the Testament of Jade, but I rolled 20 on the save (I nerfed that spell as it's broken). The Cure spells still got through (the book is caster level 20) and after they got hit with a 9-dice fireball the party wizard went on the defensive and started counterspelling its magics! Meanwhile the dwarf was hammering it with his greataxe, while the wood elf chopped it up with First Light. When he got stunned another character picked the damned weapon up. That sword went around like a hot potato whenever the wielder got stunned, and they had managed to position themselves so that only 1 of them could get hit by the mind blast.
Smart players... damn them! I tried so hard to kill one of them too as a pointed lesson for them not to pick fights with everything evil that they see, but I guess that fell flat when they killed it with only a few serious burns... 110hp's doesn't go as far as I thought (ditto the Deepspawn).
They're about to take on Castle Cormanthor now.
Sorry to hijack - I really want to read how your group took on the castle and the final encounter...!
Wasn't the group originally going to to go against the module 'Into the Dragon's Lair' next?