Attack Titan

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

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.

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.

Transformation. A Titan Shifter can use their action to transform into a Titan, in this case, the Attack 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.

Weak Point. The Titan's weak point is its nape. A creature can target the nape, which has an AC of 20 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 Attack Titan can make three attacks.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 20 (4d6 + 6) piercing damage, and the target must make a DC 18 Dexterity saving throw or be grappled by the Titan (escape DC 18). The Titan can only grapple one creature in this way, and cannot make Bite attacks while grappling a creature. 

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 24 (4d8 + 6) bludgeoning damage.

Swallow. The Titan makes a Bite attack against a creature it has grappled. On a hit, the creature takes the normal damage of the attack, and is swallowed by the Titan, ending up in its stomach cavity. While in the Titan's stomach, the creature takes 18 (5d6) acid damage at the start of the Titan's turn, has total cover against attacks outside the Titan, and is restrained.
If the Titan takes 20 or more damage from a creature inside the Titan's stomach in a single turn, the Titan must make a DC 20 Constitution saving throw or regurgitate all the creatures within its stomach, depositing them in an unoccupied space within 5 feet of it. If the Titan dies, a creature within the stomach can use 5 feet of movement to exit the corpse.

Foresight (1/Day). The Titan Shifter witnesses memories of the Attack Titan's past and future holders, gaining knowledge on how future events will play out. The Attack Titan gains advantage on all attack rolls, ability checks, and saving throws for the next 1d6 rounds. Only a Titan Shifter that has previously unlocked memories of the Attack Titan's past bearers can use this action.