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

Terrifying Presence. Each creature that isn’t a fiend within 60 feet of Zearos must succeed on a DC 17 Charisma saving throw or become frightened of it for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. A creature that makes a successful saving throw is immune to Zearos’s Terrifying Presence for 24 hours.

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

Multiattack. Zearos makes three Petrified Greatsword attacks.

Petrified Greatsword. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) slashing damage plus 11 (2d10) necrotic damage. If this damage kills a creature, it dies immediately.

Creatures hit by Petrified Greatsword are marked with Inescapable Decay. Any healing done to marked creature is absorbed and after absorbing 10 hit points worth of healing, the mark disappears.

Breath of Decay (Recharge 5–6). Zearos exhales decaying fumes in a 60-foot cone. Each creature in that area must make a DC 17 Constitution saving throw, taking 28 (8d6) poison damage on a failed save, or half as much damage on a successful one. On a failed save, the creature is also blinded until the end of Zearos's next turn. Creatures hit by Breath of Decay are marked with Inescapable Decay.

Inescapable Decay. At the start of its turn, Zearos can use his bonus action to deal 11 (2d10) necrotic damage to all marked creatures in 120ft radius centered at him.

Shadow Stride. After being hit by a ranged attack, Zearos can use his reaction to transform into intangible shadows and move up to 50ft towards the attacker without provoking opportunity attacks. Any creature he passes through takes 11 (2d10) cold damage.

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

Petrified Greatsword. Zearos makes one attack with Petrified Greatsword.

Unholy Grasp. Zearos's hand turns into shadowy tendrils and shoots out at one creature in 60ft range. That creature must make a DC 17 Dexterity saving throw or be pulled to Zearos and become grappled. Grappled creature can use its reaction to escape the grapple. Zearos can only grapple one creature at a time.