Bosses with unique mechanics. Yesterday the party fought a magically corrupted vampire tree and the man who had accidentally made it by stealing a corrupted Druid Artifact. The man had essentially infinite hp, since he was little more than a vessel for the relic at this point, and the Tree could summon plant-vampire-zombies to protect itself.
Along the way to the boss, however, the players had to deal with zombie attacks in a spooky forest. The battles were actually very underleveled for the party, but some characters would get "Marked" by blood sap.
Anytime the Vampire Tree too damage, the Marked characters took damage. When they started cleaning themselves, they realized that the damages was divided among everyone Marked, so suddenly they had to stop hurting the boss or re-mark themselves to avoid killing the low hp characters. All while fighting off weak but numerous zombies and dealing with an unkillable miniboss. And the Barb came to the adventure because he stole the artifact in the first place and the miniboss backstabbed him, left him for dead, and took it.
It was awesome. The Barbarian and Paladin had dealt over 100 damage to the Miniboss before they realized he couldn't actually die, they were just beating on a human shaped effigy. They had to balance killing the tree with not killing the Alchemist. They had to push back the zombies to get breathing room.
Oh, and the Bard came to the Adventure to find her grandfather. He was one of the zombies, and he was cognizant while trying to convince her to join the Will of the Artifact. She had to kill her own grandad and had a breakdown in game, while her player cried irl.
The characters won the fight, the players though, the players were pushed to their emotional limits.