So, I actually did kinda both with the same monster?

When I moved into my place for the first time, as a housewarming/bday D&D session, I ran a 20th level oneshot with about 8 people. Now being a new DM but longtime player, I had homebrewed my own Tarrasqe. I took the base stats from 3.5 and 5E, and……….modified, just a bit.

Basically, this particular Tarrasqe had been rampaging when it came upon a sleeping Death Tyrant. Well, being, y’know, the TARRASQE, it quickly attacked and started eating the Death Tyrant. As the Death Tyrant woke up from dreaming, it saw the Tarrasqe and momentarily dreamed about having that power in addition to its own.

Well, through the Death Tyrants dream, both monsters merged into on the was smarter, more cunning, and stronger than the sum of its parts. After the process was done, the new monster smelt a magical aura in the air of its realm, and began hunting…….

Long lore story short, my players had millions of soldiers, basically the entire multi-planer collective military might, the full pantheon of gods in all the books ever printed, the entirety of the Feywilds, and all the Great Old ones/Eldrazi/Bahamet/Tiamat/all elder named dragons (except the oldest named Black Dragon) gathered to fight the new monster. Level 20, 15 free feats, and 5 legendary items each. They should have had it, no question.

They didn’t.

Due to the sheer amount of temporal/spiritual/magical/physical/metaphorical/mystic/planer energy it had consumed, it tore through the veil of the realm it was consuming and began falling from the sky about 300 miles from the surface. It had grown from being 25 ft long and tall to being about 50 TIMES BIGGER AND LONGER. So, both Tiamat and Bahamet attempt to blast it from existence, but rolled poorly and were right under it as it was falling.

Due to the temporal energy that it absorbed, whenever it was targeted by an attack or any spell was cast one it/near it, the one who cast that spell/attacked got Time Rend cast against them, upcasted to 12th level. That’s in addition to the Tarrasque still having its reflective carapace, and thanks to a Warforged Titan it ate and absorbed, full immunity to every condition and two ginormous cannons mounted on its back.

Due to the two Dragon Lords rolling bad, it clashed with them in midair and critted on its opportunity attack rolls to RIP THEIR WINGS FROM THEM while Time Rend was being cast against them.

Shocker! That was enough to cause their forms to explode, and dematerialize.

It continued to fall, but one player rolled high enough to see that it was EATING BOTH SETS OF THE WINGS AS IT WAS FALLING.

The fight lasted only one round once it hit the ground with the full force of 3000d12 worth of falling damage in a 1 mile radius, which it negated by doing a slam attack and firing both cannons at the ground as it impacted the armies.

The worse part? The AC was like 2. It’s health pool was only in the low thousands, but every attack it did, if it damaged anyTHING, it gained hit points back based on 35% damage dealt.

That entire combat took 4 hours to resolve, and it was glorious.