Were-Terrasque (5e Race)
From D&D Wiki
This is made to be my interpretation of what this would actually be like. Probably shouldn't be used in a real game.
The legendary tarrasque is possibly the most dreaded monster of The Material Plane. It is widely believed that only one of these creatures exists, though no one can predict where and when it will strike.
A scaly biped, the tarrasque is fifty feet tall and seventy feet long, weighing hundreds of tons. It carries itself like a bird of prey, leaning forward and using its powerful lashing tail for balance. Its cavernous maw yawns wide enough to swallow all but the largest creatures, and so great is its hunger that it can devour the populations of whole towns.
Somehow this happens. It's up to you to decide how really. For me it was a party Bard. Because of course it was.
Tarrasques are gender-neutral, as such you can look however you want, and be male or female at your own discretion. Likely your physical characteristics would vary depending on the other race of the creature that mated with the Tarrasque. Somehow. I would recommend pulling names from that race instead, because the Tarrasque doesn't speak.
The destructive potential of the tarrasque is so vast that some cultures incorporate the monster into religious doctrine, weaving its sporadic appearance into stories of divine judgment and wrath. Legends tell how the tarrasque slumbers in its secret lair beneath the earth, remaining in a dormant state for decades or centuries. When it awakens in answer to some inscrutable cosmic call, it rises from the depths to obliterate everything in its path.
Ability Score Increase. Your Strength score increases by 4 and your Constitution score increases by 4. The upper cap on both these scores is 30 for you.
Age. You reach maturity as a member of you other race would, you cannot die from old age, or otherwise. When you die, after 24 hours you retreat into the earth for at least a decade. Any equipment you were carrying comes with you, and any magic items attuned to you do as well, even if not on your person. All these objects come with you. Additionally, dying could cause the Tarrasque to spawn at DM discretion.
Alignment. Up to you
Size. As a Humanoid, your size is Medium.
Speed. Your base walking speed is 30 feet. As a tarrasque your base walking speed is 40.
Blindsight. You can see anything within 120 feet of you even if you are blind, or cannot see it due to invisibility/darkness. This does not extend through walls, or creatures that are obscured.
Were-Titan. You can become a smaller Tarrasque. Abilities of this form are outlined below. You can become a Medium Tarrasque at Level 1, a Large Tarrasque at level 3, a Huge Terasque at level 5, a Gargantuan Tarrasque at level 10, a Colossal Tarrasque at level 15, and a Titan Tarrasque at level 20. You can transform once per short rest. Your Physical Stats change to be equal to the Tarrasque-the number of sizes under gargantuan, or +1 for Colossal and +2 for Titan.
Hit Dice. Increase base classes’s racial HD size up to d10 if it was lower, if not, it remains the size it was.
Natural Armor. Your AC is 18 before wearing armor or adding Dexterity. You also add your constitution modifier to AC as long as you are not wearing armor. If you don armor calculate your AC as if it had a base of 10, and use 18+Dex+Con, or the final score of the Armor+Dex, whichever is higher. This stacks with other abilities that change your AC. In those cases the 10 base given is replaced with 18. For Example: A were Tarrasque monk could use 18+Dex+Wis for its AC, or 18+Dex+Con.
Regeneration. Nonmagical weapons deal nonlethal damage to a were-tarrasque, regular, non-magical fire, acid, etc. does not harm you, nor does fall damage. You cannot drown or suffocate. A were-tarrasque has regeneration equal to its level, and it regains hit points at the beginning of its turn. This is not in effect when it is at full health
Tarrasque-Kin. The Tarrasque will not willingly harm you. If the Tarrasque is made to attack you by means magical or otherwise, it immediately makes a saving throw vs that effect with an appropriate base stat with a +8 modifier even if that effect does not normally allow, or end on a save. The DC is equal to the caster's DC. If there is no caster the DC is 15.
Strength of the World. You count as two size categories higher for determining carrying capacity while in a humanoid form. While a Humanoid you add +2 to any Strength attack or damage roll.
Elder Titan Durability. If you ever take more than 40 damage from a single source after reduction, it instead becomes 40.
Languages. You can speak, read, and write Common and one other language of your choice.
Tarrasque Size Explanation
You gain nothing from the base Tarrasque to start with, and gain what this list says you gain at the time it lists it. Pretty simple. Reference the 5th edition Tarrasque from the Monster Manual for reference.
Medium: Bite 1d6, claws 1d4, Swallow(Tiny)
Large: Bite: 1d8, claws 1d6, Swallow(Small)
Huge: Bite 1d10, claws 1d8, horns 2d6, Swallow(Medium), Siege Monster, Condition Immunities(Condition Immunities extends to Humanoid Form), additionally immune to Petrification
Gargantuan: Bite 2d6, claws 1d10, horns 2d8, Swallow(Large), Frightful Presence, Multiattack(Tail as listed)
Colossal: Bite 4d12, claws 4d8, horns 4d6, Magic Resistance
Titan: Bite 5d12, claws 4d12, horns 6d6, tail 4d6, Reflective Carapace
Random Height and Weight
In Medium form likely determined by your other race. As a Tarrasque, likely from 1 to 100's of tons depending on how big you are at the time. Height determined based on your size. (If you're huge(3x3) Likely you're also about 15+ feet tall.
I would recommend pulling form elsewhere for these, probably from the other half of your race. I can't be f*cked to make them, but some fun ideas off the top of my head are an appetite for destruction, or wanting to find your Mom/Dad/Whatever role the Tarrasque filled in this blasphemy that is you.