Blackheart (Mythic Trait; Recharges after a Short or Long Rest). When Zylvaran is reduced to 200 hit points or fewer, his black heart pulses with negative energy, filling the dracolich with ungodly strength; the beating of the heart reveals it within his otherwise empty, cavernous rib cage. Zylvaran regains 200 hit points as the thrum of anti-life surges through him, causing his eyes to glow brighter and black tendrils of energy to limn his form. While the heart remains in its crystal reliquary Zylvaran cannot be reduced to 0 hit points. The reliquary has an AC of 25, 250 hit points, and resistance to all damage types other than necrotic, poison, and psychic, to which it is immune; additionally, it is immune to all nonmagical attacks. It is immune to all conditions, and all of its ability scores count as 10 (+0) for the purposes of making saving throws. The reliquary itself counts as a ring of spell turning, having an uncanny reflective ability.

Undead Nature. Zylvaran no longer requires air, food, drink, or sleep.

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

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

Promise of Death. Zylvaran's body is suffused with virulent necrotic energies, a result of his wizardry. Each melee attack done by him deals an additional 3d6 necrotic damage, which is included with the attack.

Rejuvenation. If he has a phylactery, the destroyed Zylvaran gains a new body in 1d10 days, regaining all of his hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Spellcasting. Zylvaran is a 23rd-level spellcaster. His spellcasting ability is Intelligence (spell save DC 23, +14 to hit with spell attacks). The dracolich has the following wizard spells prepared:

Cantrips (at will): blade ward, control flames, dancing lights, infestation, prestidigitation

1st level (5 slots): absorb elements, detect magic, shield

2nd level (5 slots): detect thoughts, dragon's breath, mirror image

3rd level (5 slots): counterspell, dispel magic, haste, lightning bolt, slow

4th level (4 slots): black tentacles, blight, greater invisibility, sickening radiance

5th level (4 slots): mislead, wall of force

6th level (4 slots): circle of death, disintegrate, mental prison, scatter, soul cage

7th level (3 slots): finger of death, forcecage, power word pain

8th level (3 slots): Abi-Dalzim's horrid wilting

9th level (2 slots): foresight, time stop

Turn Resistance. Zylvaran has advantage on saving throws against any effect that turns undead.

Multiattack. Zylvaran can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 9 (2d8) acid damage plus 10 (3d6) necrotic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage plus 10 (3d6) necrotic damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage plus 10 (3d6) necrotic damage.

Frightful Presence. Each creature of Zylvaran's choice that is within 120 feet of the him and aware of him must succeed on a DC 22 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Zylvaran's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). Zylvaran uses one of the following breath weapons.

Acid Breath. Zylvaran exhales acid in a 300-foot line that is 20 feet wide. Each creature in that line must make a DC 25 Dexterity saving throw, taking 90 (20d8) acid damage on a failed save, or half as much damage on a successful one.

Mortiferous Breath. Zylvaran exhales a gushing, black gout of tar-like liquid in a 90-foot cone. Creatures caught in the area must make a DC 25 Constitution saving throw or take 55 (10d10) necrotic damage. If a creature fails this save by more than 10 (i.e. a 14 or lower) they are reduced to 0 hit points. If there are any Large or smaller corpses in the area of effect, they rise as skeletons or zombies (subservient to Zylvaran) as appropriate.

Omniplegia Breath. Zylvaran exhales a stinging, yellowish gas in a 90-foot cone. Each creature in that line must make a DC 25 Constitution saving throw or be paralyzed for one hour. Creatures can repeat this save at the end of each of their turns, ending the effect on a success.

Zylvaran can take 4 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. Zylvaran regains spent legendary actions at the start of its turn.

Cantrip. Zylvaran casts a cantrip.

Detect. Zylvaran makes a Wisdom (Perception) check.

Tail Attack. Zylvaran makes a tail attack.

Wing Attack (Costs 2 Actions). Zylvaran beats its wings. Each creature within 15 feet of the dracolich must succeed on a DC 26 Dexterity saving throw or take 16 (2d6 +9) bludgeoning damage and be knocked prone. Zylvaran can then fly up to half his flying speed.

Tyrant of the Waste (Costs 3 Actions). Zylvaran flares his wings wide, spreading negative energy out in waves around him. Each non-undead creature within 60 feet of him must make a DC 25 Constitution saving throw or take 42 (12d6) necrotic damage, or half as much on a successful save. Any humanoid corpses in the area rise as skeletons or zombies under the dracolich's control.

If Zylvaran's mythic trait is active, he can use the options below as Legendary Actions within 1 hour of using Blackheart.

Dark Pulse. The dark heart in his breast pulses powerfully, each non-undead creature within 20 feet of Zylvaran takes 21 (6d6) necrotic damage.

Perverse Vitality (Costs 2 Actions). Zylvaran incants a twisted, dark language in tune with the now palpable beating of his heart. He regains spell slots with a cumulative level of 9 (2d8).