The Pestilence. Zytheles spreads an abhorrent infection wherever he goes. This infection turns the land into a lifeless, grey expanse coated in a film of bubbling, caustic goo seeping up through the ground, a horrid imitation of his layer of the abyss. The infection spreads passively at a speed of 15 feet per day, but can be cleansed by fire or radiant damage.

Unyielding Essence. Zytheles is immune to any spell or effect that would alter his form, as well as those that would read his thoughts, determine if it he lying, or magically influence his thoughts or behavior.

Harbinger of Disease. Zytheles can call upon the latent power of the pestilence to vanquish his foes. Every time he hits with an attack, he deals an additional 1 damage of that attack's specified damage type per square mile of pestilence in existence. Additionally, he gains 4 hit-points per square mile of pestilence in existence.

Legendary Resistance (2/Day). If Zytheles fails a saving throw, he can choose to succeed instead.

Caustic Gas (Acid). Zytheles roars a bone chilling streak, spewing forth a wave of searing white gas from his gaping maw. Each creature in a 50-foot cone must make a DC 20 Constitution saving throw, taking 28 (2d12+15) poison damage plus 13 (4d8) acid damage on a failed save, or half as much damage on a successful one.

Entropic Infusion (Necrotic). Zytheles accelerates entropy on a target he can see within 50 feet of him. The target must succeed on a DC 20 Constitution saving throw, taking 40 (4d12 + 14) cold damage plus 27 (2d12 + 14) necrotic damage on a failed save, or half as much damage on a successful one.

Cryonic Claw (Cold). Melee Weapon Attack: +17 to hit, reach 4 ft., 1 target. Hit: 57 (4d20+15) slashing damage plus 31 (2d12 + 18) cold damage. 

Horrid Appendage (Acid). Melee Weapon Attack: +15 to to hit, reach 30 ft., 1 target. Hit:  21 (4d6+7) acid damage and the target is Grappled (escape DC 21). Until the grapple ends, the target is Restrained, and has disadvantage on Strength checks and Strength saving throws. 30 radiant or fire damage can be used to end the grapple, though this damage will not carry over to Zytheles.

Zytheles 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. Zytheles regains spent legendary actions at the start of his turn.

Soul Sap. Zytheles targets one creature he can see within 30 feet of him and attempts to siphon its soul, reducing its Strength or Constitution by 1 (Zytheles' choice). This ability cannot be used on the same creature twice in the span of 1 round. Lost Strength and Constitution points are regained after a long rest. If a creature's Strength or Constitution reaches 0 it dies and its soul is obliterated.

Lunge. Zytheles lunges up to 50 feet to an unoccupied space he can see. If he lunges into a creature, that creature must make a DC 17 Dexterity saving throw or take 21 (2d20) bludgeoning damage and be stunned for 1 round.

Reform (Costs 2 Actions). Zytheles call upon the power of the pestilence to mend his broken body, gaining 20 hit-points, plus 1 for every square mile of pestilence in existence. If he has taken radiant or fire damage since his last turn he only heals for half of this amount.