The Second Hiss (Mythic Trait; Recharges after a Short or Long Rest). When the Dracolich is reduced to 0 hit points, it doesn't die or fall unconscious. Instead, it's bones fall apart, revealing it's core. The bones of the dracolich then form into the shape of a basilisk around the core creating a new creature that has an AC of 20 and 90 hit points. It is immune to bludgeoning, piercing, and slashing damage from nonmagical attacks, and it is immune to all conditions. 

Legendary Resistance (3/day). If the dracolich fails a saving throw, it can choose to succeed instead.

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

Rejuvenation. If it has it's core, the destroyed dracolich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the core.

Runic Carapace. Any time the dracolich is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the dracolich is unaffected. On a 6, the dracolich is unaffected, and the effect is reflected back at the caster as though it originated from the dracolich, turning the caster into the target.

Multiattack. The dracolich can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws. It can use its Swallow instead of its bite.

Lightning Breath (Recharge 5–6). The dracolich exhales a web of lightning in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 70 (20d6) Lightning damage on a failed save, or half as much damage on a successful one.

Swallow. The dracolich makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the dracolich, and it takes 56 (16d6) necrotic damage at the start of each of the dracolich's turns.

If the dracolich takes 40 damage or more on a single turn from a creature inside it, the dracolich must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the dracolich. If the dracolich dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

The dracolich 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. The dracolich regains spent legendary actions at the start of its turn.

Chomp (Costs 2 Actions). The dracolich makes one bite attack or uses its Swallow.

Move. The dracolich moves up to half its speed.

Detect. The dracolich makes a Wisdom (Perception) check.

If the dracolich's mythic trait is active, it can use the options below as legendary actions until it's Rejuvenation property activates.

• Swipe. the basilisk makes two attacks with it's claws.

• Fire breath (Costs 3 Actions). The basilisk recharges it's Fire Breath and uses it.

Additionally, the basilisk loses it's Magic resistance trait and it is instead replaced by two new ones.

Runic Carapace. Any time the basilisk is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the dracolich is unaffected. On a 6, the dracolich is unaffected, and the effect is reflected back at the caster as though it originated from the dracolich, turning the caster into the target.

Petrifying Gaze. If a creature starts its turn within 30 feet of the basilisk and the two of them can see each other, the basilisk can force the creature to make a DC 21 Constitution saving throw if the basilisk isn't incapacitated. On a failed save, the creature magically begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.