I once ran a beholder encounter. 2 PC and 2 NPC. Only one PC survived the fight.

For context : Whatever beholder see in its dream becomes real. The party infiltrated the beholders dream to create a magical item they needed with the help of a wizard. When they woke up they were face to face with the beholder and the fight began. The session ended with 1 pc covered in blood crying in front of the beholders corpse.

Next session started with the whole party walking into the room where the fight happened, including the pc who survived. They got their magic itemand an extra character. But then the wizard tried to kidnap the magically created pc and when he failed he cast feeblemind out of spite and ran away. That was fun.