A nailbiter climax to the Svardborg frost giant adventure from Storm King's Thunder. In my version, the giants are aided by the Zhentarim and have conquered Waterdeep. The players have stepped into the role of some supporting characters to raid Svardborg, which currently sits in Waterdeep's harbor. These characters are all agents of Force Grey under the command of the Blackstaff. Effectively, they're the resistance to the occupation.
In the last session, the frost giant jarl's white dragon tore to pieces the goliath barbarian PC. The gnome artificer PC took refuge inside a rope trick but was quickly encircled by waiting giants. And the other PCs fled into a tunnel deep within the iceberg for a short rest.
The giants discovered where the characters were hiding and sent in a pack of yetis to flush them out. Yetis are actually an awesome boss monster for low level characters. But the aasimar bard, acting as rearguard, swiftly wiped them out with erupting earth and wall of fire, which collapsed the tunnel behind him.
The other characters exited from the other end of the tunnel, which emerged into the wreckage of a giant ship at the center of Svardborg. Naturally, the giants knew the location of the tunnel exit. The jarl, Storvald, swooped in on his white dragon and alighted upon the ship's mast. Four more frost giants converged from across the ice. Battle ensued.
The warforged cleric had used the rest to cast summon planar ally. We fudged the result and agreed that the coolest thing that could appear would be a steel predator. They are savage! Or, as one of my players said after reviewing its stat block, "I wouldn't want to fight this guy." It was immune to all the attacks of the giants and was a total gamechanger for the outcome of the fight.
The first thing the steel predator did was use its roar to stun Storvald.
The white dragon breathed on the warforged cleric and the halfling wizard, knocking out the latter. Then it took wing to get some distance from the players.
The cleric got the wizard back on her feet. But she was dropped again by a rain of boulders from the frost giants. A giant closed for melee and chopped her in half with its axe.
The other giants surrounded the cleric with a flurry of blows from their greataxes and he soon fell.
The gnome artificer popped out of her rope trick, dodged several boulders from frost giants, and got the cleric back up with a healing spell.
The steel predator used dimension door to teleport to the dragon's back. The dragon knocked the steel predator prone with wing attack, sending the predator falling away from the dragon. But as it fell the steel predator cast plane shift, banishing the dragon to the mechanical plane of Mechanus. Forever.
Storvald plummeted from the sky and landed on the deck of the ship. One of the players was controlling Harshnag, the good frost giant. In my Realms, Harshnag and Storvald are estranged brothers. This was their showdown for control of the tribe.
Storvald dropped the bard, then turned and attacked Harshnag. However, the other characters focused their fire on Storvald. He was hit with eldritch blast, flame strike, and more. Harshnag landed the killing blow. When Storvald fell, the mantle of leadership passed to Harshnag, and the frost giants lowered their weapons and knelt before him.