Vacuous. Jeff can devour any amount of matter without getting full.

Already Perfect Form. Jeff is immune to any spell or effect that would alter his form.

Magic Resistance. Jeff has advantage on saving throws against spells and other magical effects. 

Destructive Nature. Jeff's Armor Class includes his Strength Modifier.

Healing Clay. For each pound of clay Jeff eats, he regains 55(10d10) hitpoints.

Hoover. Jeff always hovers 10 feet above the ground, allowing him to move into other creatures' squares.

Multiattack. Jeff makes one Devour or Ram attack, then uses Regurgitate. 

Devour. Melee Weapon Attack: +19 to hit, reach 0 ft., all targets in his square(s). Hit: 65 (10d10 + 10) bludgeoning damage and 55 (10d10) acid damage. Also if the attack hits, the target is grappled (Escape DC 20), and must succeed on a DC 20 Dexterity saving throw at the start of Jeff's turn or it takes 65 (10d10 + 10) slashing damage from the whirling blades in his stomach. While a creature is grappled in this way it takes 55 (10d10) acid damage per turn, Jeff cannot use his hovering speed, and his Hoover trait is not active.

Ram. Melee Weapon Attack: +19 to hit, reach 5 ft., 1 target. Hit: 65 (10d10 + 10) bludgeoning damage, and the target must succeed on a DC 20 Strength saving throw or be pushed 30 feet away and knocked prone.

Regurgitate. Jeff launches a grappled target 50 feet into the air and 25 feet away, dealing  17 (5d6) bludgeoning damage from the fall, and is covered in acid until they take an action to scrape the acid off. The acid deals 55 (10d10) acid damage per turn.

Jeff can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Jeff regains spent legendary actions at the start of his turn.

Hoover. Jeff moves up to two times his speed.

Ram. Jeff uses Ram.

Devour. Jeff moves up to his speed and makes one Devour attack.

Seek and Destroy (Costs up to 3 Actions). Jeff moves up to his speed towards a creature he can see, then makes a Ram attack. He then repeats the process 1 more time for each additional legendary action he took.

Jeff can take 5 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature’s turn. Jeff regains spent mythic actions at the start of his turn. He can only use mythic actions if he has less than half his hitpoints left.

Enraged Hoover. Jeff moves up to 3 times his speed.

Enraged Devour. Jeff moves up to his speed and makes a Multiattack.

Enraged Ram. Jeff makes two Ram attacks.

Enraged Seek and Destroy (Costs up to 5 Actions). Jeff moves up to his speed towards a creature he can see, then makes two Ram attacks. He then repeats the process 1 more time for each additional mythic action he took.