Crystal Form. Melee attacks that strike Zerelith cause the attacker to take 10 (3d6) radiant damage as shards of crystal explode outward.

Lair Awareness. While in its lair, Zerelith can sense vibrations through the ground within 1 mile and knows the location of any creature touching the ground.

Shimmering Hypnosis (Recharge 5–6). Zerelith can radiate dazzling lights from its crystalline scales. Each creature within 30 feet that can see it must succeed on a DC 18 Wisdom saving throw or be charmed for 1 minute. While charmed, a creature is incapacitated and has a speed of 0.

Multiattack. Zerelith makes three attacks: one with its bite and two with its constrict.

Bite. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 25 (3d12 + 6) piercing damage and 10 (3d6) radiant damage.

Constrict. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage. The target is grappled (escape DC 18). Until this grapple ends, the target is restrained, and Zerelith can’t constrict another target.

Prismatic Breath (Recharge 5–6). Zerelith exhales a cone of blinding light and crystal shards in a 60-foot cone. Each creature in the area must make a DC 19 Dexterity saving throw, taking 55 (10d10) radiant damage on a failed save, or half as much on a success. Creatures that fail their saving throw are also blinded for 1 minute. A blinded creature can make a DC 18 Constitution saving throw at the end of each of its turns, ending the effect on itself on a success.

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

Shatter Pulse. Zerelith emits a burst of sound and magic in a 10-foot radius. Each creature in the area must make a DC 17 Constitution saving throw or take 15 (3d8) thunder damage and be deafened for 1 minute.

Crystal Spike. Zerelith causes crystal spikes to erupt from the ground in a 20-foot radius within 60 feet. Creatures in the area must make a DC 18 Dexterity saving throw or take 22 (4d10) piercing damage and are restrained by the spikes until the end of their next turn.

Reflective Barrier. (Costs 2 Actions) Zerelith raises a shimmering wall of light that grants it resistance to all damage until the start of its next turn. Spells that target it directly are reflected back at their caster unless the caster succeeds on a DC 17 Intelligence saving throw.