Playing the haunted house in Curse of Strahd, and our party was tracking down "baby Walter" in the sub-basement.
We made it to a sacrificial alter, when a bunch of ghostly cultists appeared and demanded "One must die. One must die."
We have a magic using zombie in the party, which was hanging back in the hallway. The cast disguise self to look like a living person, got on the altar for one of the other party members to run them through with a sword. We figured the zombie could be the most convincingly dead, and even if the sword did damage it wouldn't for real kill them. For the deception check the DM had the zombie player roll, they got a nat 20, so the ruse worked and the zombie didn't take any damage at all. The table went wild at this.
The specters disappeared, and Walter appeared from the murky water.
As a human fighter, I jumped on his back, hanging on by digging one hand axe in, and stabbing at the tendrils holding the baby in place with the other.
The rest of the party tried o damage Walter. The zombie player tried to run past but got absorbed into Walter.
Everyone kept hacking at the Walter monster until finally our large half Storm giant monk, who had dropped his weapon, was able to reach in with his bare hands and rip baby Walter out of the core of the monster, causing the creature to collapse into gore and Walter to turn into dust.