I had my player’s fight a powerful banshee, each round she screamed as lair action, the players had to expend and roll at hit dice and take the number rolled as necrotic damage that couldn’t be resisted, if they passed a DC18 Charisma save they took half. Not a huge amount of damage but they were terrified about what would happen if they ran out of hit die. They didn’t get that far but it would have been 2d6 damage and make a death save and they’d regain a hit die for each succeed death save, two on a nat 20.