The tarrasque is the iconic campaign ending D&D monster. It’s almost a joke or a meme at this point, where the tarrasque is a “and then everyone died” punchline. It’s often the first big monster that new players find that gets them excited about one day fighting this gigantic kaiju. This is pretty ironic, since the tarrasque throughout the editions has always been lackluster. Usually bested by silly tactics, and depending on the edition, beaten by level 1-5 PCs easily enough.
In 3, 4, and 5e, the tarrasque seems to consistently have similar problems. It’s often slower than horses, and a squad of horse archers can beat it. If it can catch horses, flying archers beat it. 5e gets around those with its invulnerability to non-magic weapons, which IMO is a mechanic I dislike. It also only sort of gets around it, since 5e doesn’t have magic item guidelines, leaving the tarrasque’s beatability firmly in the “ask your DM” realm. If a kingdom can come up with a dozen pegasus mounted, magic longbow knights for defenders, they can eventually kill it while it tries to run away in a super anti-climatic battle. 3e got around that by requiring a Wish spell to prevent its regeneration, but 3e also made it pretty easy to get wish spells, especially by campaign capstone time. 4e had some force people to stop flying power, but it was very awkward in practice. Also, you could just fly higher than its aura and be fine. And since it’s not smart to be near it in melee, why wouldn’t you fly at max bow range and shoot it?
Lore wise, the tarrasque is a kaiju that erupts from the earth and ruins a city. The 5e tarrasque is close to actually being effective at what it does, but we are going to add a couple tweaks to abilities based on lore to help it out. These will make it more effective at its job and might make it a little harder in some places, but I would not adjust its CR (I would say RAW is too high).
5e in particular has one glaring weakness for the tarrasque. In 5e armies and masses of troops are the way to go. Even just summoning a couple of extra minions can turn the tide of a battle. Even with legendary actions, the tarrasque is behind in action economy vs an army. Especially if a DM gives some minor magic swords to the army. Sure, the tarrasque is dropping someone with each of its 8ish attacks a round, but an army has a couple hundred or thousand soldiers.
Unkillable. The first change we are implementing is straight forward. The tarrasque is supposed to be unkillable. We are giving it massive regeneration and adding back the 3e wish spell requirement. In 5e, a wish spell is a lot bigger of a deal, and our tarrasque is actually unkillable without it. Otherwise, it just continually regenerates.
Erupts from the earth. The tarrasque is supposed to hibernate way below the surface of the earth, and then burst free once every thousand years to ruin some major city. We are giving it back its prior edition burrow speed. Furthermore, we are giving it an eruption power. If it bursts from the ground near you, you will be showered in debris. When combined with its siege monster power, this thing will be decimating cities.
Weak to fliers. The tarrasque is always weak to flyers. That said, the above burrow power means you can’t just chase it and pepper it with arrows until it dies. If it’s actually feeling threatened, it can just hide underground for a minute or two and come back fully rested and ready to ramage again. It can just ignore the pegasus guards that have defeated it for generations.
Breath weapon. It’s a godzilla. It gets a breath weapon. Ours shoots a line of lightning, because again, it’s a godzilla. This can kind of help with fliers, but not really (they can probably just fly higher). It’s mostly just cool.
Ignore armies. Raising an army shouldn’t beat the tarrasque. So it doesn’t anymore. We have the eruption power, which is a new AoE and generally blows everyone, or everything up as it moves around (a cheesy DM might even have the tarrasque travel like a streetshark to cause constant eruptions against an army). It also has the AoE breath, as a legendary action, mowing down large swaths of soldiers. Lastly, we are changing three mechanics. Instead of multiattack, we are giving it “Sweeping attack”. It can make one attack with its bite, horns, and tail like normal, but now it can also make a claw attack against everyone it can reach. It’s a blender for your troops, meant to simular one arm just sweeping through the line. We also give its bite the ability to swallow folks faster, so it’s just gulping down your war elephants. Lastly, its fear aura is modified. Anyone who sees it must make a save. Goodby low level archers and armies. If you’re within 120ft, you get disadvantage. We also take away the one effect per day. If you can’t save vs it, and you see the tarrasque again, you’re scared again.
The army ignoring powers sound like a lot, but ironically, they don’t help it much vs PCs. There are only so many PCs in a party, and there isn’t much a difference between “make 8 attacks against whoever you want” and “make 4 attacks against whoever you want, and one against everyone”.
Kill anything. Due to a weird 5e design quirk, the tarrasque can’t actually kill a decent amount of things due to immunity to non-magic weapons. This is an easy fix. The tarrasque’s weapons are now considered magical.