Tarrasque

A tarrasque as ally is the most absurd framing in the bestiary, and you have to commit to the absurdity completely or skip it. The cleanest version is a wish-spell mistake or a divine intervention gone literal. The party prayed for "a champion that cannot be stopped", or wished for "the strength to break the wall", and the deity or genie answered correctly and unhelpfully. The tarrasque appears, walks toward the named obstacle, and does what tarrasques do. The party is now responsible for everything in its path.

Mechanically the tarrasque is not steerable. It has Int 3 and the Unaligned alignment line, so it does not understand commands beyond the original direction. Point it at a fortress with Siege Monster (double damage to objects and structures) and the wall comes down in two rounds. After the wall, it keeps walking. The party has to outrun it, distract it, or wait for the wish duration to expire. The horror of the scene is that the party is now the thing trying to manage collateral damage, and there's a town two miles past the fortress.

In any actual combat, the tarrasque does what it does. Bite, Claw, Tail, Swallow, Bellow on recharge, all aimed at whatever is in front of it. If enemies are in front, great. If a panicked merchant convoy is in front, also great, from the tarrasque's point of view. The party's job is to stay behind it and try to redirect with explosions, with summoned terrain, with a giant mirror, with anything that smells like food in another direction. None of these will fully work.

Set the duration as a hard timer the party can see (an hourglass made of starlight, a candle the cleric is holding, a mark on the wizard's hand that fades). The drama is the countdown, not the fight.

The tarrasque is a campaign event, not an encounter. Run it that way. By the time the party can survive a hit from the thing, they should already know its name, have seen the path of its last waking, and have spent at least one session arranging the means to fight it. CR 30, 697 HP, AC 25, immunity to Charmed and Frightened, Magic Resistance, six Legendary Resistances per day, and Reflective Carapace that bounces ranged spells back on a 6. There is no clever trick. The party kills it with attrition and prep, or it kills them.

Open with Thunderous Bellow if it's available. DC 27 Con save in a 150-foot cone, 12d12 thunder on failure, average 78, plus Deafened and Frightened. Most parties lose a member outright on a failed save. After it's spent, the tarrasque uses Multiattack: one Bite at +19 for 4d12+10 piercing plus Grappled (escape DC 20) and Restrained, then three Claw or Tail attacks in any combination. Bite is the priority; once a target is grappled, Swallow as a bonus action sends them inside on a failed DC 27 Strength save for 16d6 acid per turn.

Legendary actions are the rhythm. Onslaught at the end of a damage-dealer's turn lets you pile a Claw or Tail on top of regular round economy. World-Shaking Movement breaks Concentration in a 60-foot emanation and knocks Medium-or-smaller creatures Prone. Use it once per round whenever a wizard is concentrating on something the party needs. The combo of Concentration loss plus Bite plus Swallow is the kill loop, and anyone who falls Prone next to a tarrasque with reach 30 ft. will not get up cleanly.

Reflective Carapace shapes the magic plan. Magic Missile and Disintegrate both bounce on a 6, which means the wizard's Disintegrate is a coin flip on whether it kills the wizard. Tell the party this exists before they cast. The party should learn to land melee, save spell slots for area buffs, and keep a healer in range of the front line who is being chewed.

The tarrasque does not flee; it eats until it sleeps. Have it bellow once over a city skyline three sessions before the fight. The shape on the horizon should be a known thing.