Foul. Any creature  that starts its turn able to see Zargon must succeed on a DC 21 Constitution saving throw or be frightened until the start of the creature’s next turn.

Legendary Resistance (3/Day). If Zargon fails a saving throw, it can choose to succeed instead.

Magic Resistance. Zargon has advantage on saving throws against spells and other magical effects, and is immune to divinity spells.

Regeneration. Zargon, until his horn is removed, regenerates 5d20 at the beginning of each of his turns.

Removing the horn: You can take an action to do a DC 25 strength check. You take 5d20 necrotic damage, whether you succeed or fail.

Siege Monster. Zargon deals double damage to objects and structur

Multiattack. Juiblex makes three Acid Lash attacks.

Tentacles. Melee Weapon Attack: +4 to hit, reach 10 ft., one creature. Hit: 7 (1d10 + 2)piercing damage, and the target must succeed on a DC 11 Constitution saving throw or be poisoned for 1 minute. The poisoned target is paralyzed, and it can repeat the saving throw at the end of each of its turns, ending the effect on a success.

The target is also grappled (escape DC 15). If the target is Medium or smaller, it is also restrained until this grapple ends. While grappling the target, the grell has advantage on attack rolls against it and can’t use this attack against other targets. When the grell moves, any Medium or smaller target it is grappling moves with it.

Acid Lash. Melee or Ranged Weapon Attack: +14 to hit, reach 10 ft. or range 60/120 ft., one target. Hit: 21 (4d6 + 7) acid damage. Any creature killed by this attack is drawn into Juiblex’s body, here the corpse is dissolved after 1 minute.

Eject Slime (Recharge 5–6). Juiblex spews out a corrosive slime, targeting one creature that it can see within 60 feet of it. The target must succeed on a DC 21 Dexterity saving throw or take 55 (10d10) acid damage. 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 mending spell.