US

Astral Implosion. If US is reduced to 0 Hit Points, it implodes and ejects any creatures and objects engulfed by it into the Astral Sea. US itself vanishes, leaving behind a layer of slime on everything that was within 600 feet of it. In 1d20 years, US reconstitutes on a random world in the Material Plane.

Foul. Constitution Saving Throw: DC 26, any creature (other than an Ooze) that starts its turn in a 10-foot Emanation originating from US. Failure: The target has the Poisoned condition until the start of its next turn. 

Legendary Resistance (5/Day). If US fails a saving throw, it can choose to succeed instead.

Magic Resistance. US has Advantage on saving throws against spells and other magical effects.

Regeneration. US regains 30 Hit Points at the start of its turn. If it takes Fire or Radiant damage, this trait doesn't function at the start of its next turn. Juiblex dies only if it starts its turn with 0 Hit Points and doesn't regenerate.

Royal Resplendence (Mythic: Recharges after a Short or Long Rest). If US is reduced to 0 Hit Points, it instead is reduced to 1 Hit Point as it undergoes a regenerative apotheosis. It immediately regains 585 Temporary Hit Points and 35 Tendrils erupt from US, extending up to 120 feet away. While these tendrils exist, US has Immunity to Fire and Radiant damage. Whenever US suffers at least 10 points of damage in a single turn, a tendril is destroyed. 

If US' Hit Points are restored to 585, any remaining tendrils retract and Royal Resplendence recharges immediately. 

Spider Climb. Juiblex can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. US makes three Pseudopod attacks and uses Engulf. It can replace one attack with a use of Restraining Glob or two attacks with Eject Slime.

Pseudopod. Melee Attack Roll: +17, reach 30 ft. Hit: 44 (6d10 + 8) Force damage.

Engulf. US moves up to its Speed and can move through the spaces of Huge or smaller creatures and objects. Strength Saving Throw: DC 25, each creature or object whose space US enters for the first time during this move. Failure: The target is engulfed. While engulfed, a target has Total Cover against attacks and other effects outside US, and when US moves, the engulfed target moves with it. A nonmagical object is destroyed after spending 1 minute engulfed.

While engulfed, a creature takes 21 (6d6) Force damage at the start of each of its turns, is suffocating, has the Restrained condition, and repeats the save at the end of each of its turns. An engulfed creature that is reduced to 0 Hit Points dissolves into ash, which is ejected into the Astral Sea.

Success: The target escapes and enters the nearest unoccupied space.

Restraining Glob. US lobs a slimy glob at one Large or smaller creature it can see within 600 feet of itself. Dexterity Saving Throw: DC 25, the targeted creature. Failure: 29 (6d6 + 8) Acid damage. The glob rolls the target 60 feet straight toward US, and the target has the Restrained condition until the end of its next turn, when the glob harmlessly dissolves. Success: Half damage only.

Eject Slime (Recharge 5–6). Dexterity Saving Throw: DC 26, one creature US can see within 60 feet of it. Failure: 88 (16d10) Acid damage. Success: Half damage. Success or Failure: Unless the target avoids taking all of this damage, any metal armor worn by the target takes a permanent −1 penalty to the AC it offers, and any metal weapon the target is carrying or wearing takes a permanent −1 penalty to damage rolls. The penalty worsens each time a target is subjected to this effect. If the penalty on an object drops to −5, the object is destroyed. The penalty on an object can be removed by the [Tooltip Not Found] spell.

Legendary Action Uses: 3. Immediately after another creature’s turn, the blob can expend a use to take one of the following actions. The blob regains all expended uses at the start of each of its turns.

Decay. US deals 14 (4d6) Necrotic damage to each creature engulfed by it. US can’t take this action again until the start of its next turn.

Grasping Glob. US uses Restraining Glob. US can’t take this action again until the start of its next turn.

Lashing Goop. US makes one Pseudopod attack.

If US is a Mythic encounter, it can choose from the options below as additional Legendary actions while it possesses Temporary Hit Points from its Royal Resplendence and has at least one tendril still extended. 

Move Tendril. US chooses an unoccupied space within 120 feet. A tendril lands at that location and splashes nearby spaces with acid. All creature and objects within 5 feet of that point take 14 (4d6) Acid damage. All terrain in a 5-foot Radius circle centered on the point becomes Difficult Terrain. 

Recharge Slime. US attempts to recharge its Eject Slime trait. 

Slap. US makes one Pseudopod attack. The target must then make the following saving throw. Strength Saving Throw: DC 25. Failure: The target is pushed up to 30 feet away from US.