Follow-up from OP
So we played last night. The party decided to explore the rest of the lair. There seemed to be no good reason to throw random encounters at them, so they took their time. They eventually defeated the bonedrinkers (though they certainly did damage to the party, nearly killing two of them) and took the time to destroy the Lion's Heart. They used magic weapons and mighty sword arms - I ruled that it took 2 hours and that the two attacking it were both exhausted afterwards. They left the lair and camped some distance away, and my dice mercifully did not roll an attack on them during their sleep.
But about the Ghostlord: after much talk, they decided to leave well enough alone and get the hell out of the Thornwaste and head back to Brindol (good thing too - they will make it to the city with about only a day to spare). The phylactery is still in one piece, and they really don't know how to destroy it. As they travel back to Brindol (on foot, no less!), at the end of the day, if the evoker has spells to spare, he's going to launch them at the phylactery. Maybe it will be destroyed, maybe not. But in any case, it looks like, sometime, down the road, they will have to face the GL again. And he'll be much better prepared next time!
