Colossal Titan

Curse of Ymir. The Titan Shifter possessing this Titan will die 13 years after gaining the Armored Titan.

Explosive Transformation. A Titan Shifter can use their action to transform into a Titan, in this case, the Colossal Titan. The Titan keeps the Titan Shifter's Intelligence, Wisdom, and Charisma scores (the above scores are placeholders), as well as the Shifter's alignment. When the Titan dies, or when the Shifter takes an action to end the transformation, the Shifter is ejected from the Titan's body, which instantly drops to 0 hit points and begins evaporating, unless the Shifter rejoins their Titan body within the next minute.
When the Titan Shifter transforms, they can choose to release a massive amount of heat, akin to an explosion, as they transform into the Titan. Every creature within 1,000 feet of the Colossal Titan must make a DC 20 Dexterity saving throw, taking 65 (10d12) fire damage (minus 1d12 per 100 feet away from the center of the radius the creature is), or half on a success. All structures within 500 feet of the Colossal Titan are completely destroyed, and all structures within 1,000 feet of the Colossal Titan are heavily damaged.

Immense Heat. The Colossal Titan gives off an extreme amount of heat. A creature that ends its turn within 10 feet of the Colossal Titan takes 7 (2d6) fire damage.

Regeneration. As long as the Titan has 1 hit point, it regains 20 hit points at the start of its turn. The Titan Shifter within the Titan's body doesn't benefit from this trait, but regrows any lost body parts while inside the Titan.

Siege Monster. The Armored Titan deals double damage against objects and structures.

Titan Steam. A Titan gives off a large amount of heat, and it produces steam around its wounds as it regenerates. The area around any open wounds the Titan has are lightly obscured by this steam, and upon death, the Titan's corpse is moderately obscured until the entire corpse evaporates.

Weak Point. The Titan's weak point is its nape. A creature can target the nape, which has an AC of 19 and 30 hit points. If the attack against the nape misses, it misses the Titan altogether. The nape does not benefit from the Titan's regeneration trait, and the Titan dies if the nape is destroyed. Any damage done to the nape is done to the Titan Shifter within the Titan's body. If the Titan Shifter dies, the Titan dies.

Multiattack. The Colossal Titan can make two attacks.

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 29 (6d6 + 8) piercing damage, and the target must make a DC 20 Dexterity saving throw or be grappled by the Titan (escape DC 20). The Titan can only grapple one creature in this way, and cannot make Bite attacks while grappling a creature. 

Slam. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 33 (6d8 + 8) bludgeoning damage.

Steam Emission (Recharges on a Short or Long Rest). The Colossal Titan can begin emitting extremely hot steam in a 100 foot radius. Each creature that starts its turn in the radius must make a DC 20 Constitution saving throw, taking 33 (6d10) fire damage and being pushed 50 feet away from the center of the radius on a failed save, or half damage and being pushed 25 feet from the center of the radius on a successful save. The area within the steam is considered difficult terrain for ODM gear, and cannot be traversed on foot.
The emission lasts for 1 minute, but to extend the emission beyond that, the Colossal Titan begins taking the same damage the steam deals, ignoring the Titan's immunity to fire damage. If the Titan is reduced to 0 hit points, the emission ends and the Titan dies.