Adamantine Armor. Any critical hits made against Zilliax are regular hits instead. 

Alert. Zilliax can't be surprised while you are conscious, gains a +5 bonus to initiative and other creatures don't gain advantage on attack rolls against Zilliax as a result of being unseen by Zilliax.

Lightning Rod. Whenever any form of lightning type attack, area of effect spell or lair effect that involves lightning damage within 20 ft of Zilliax or targeting Zilliax, Zilliax will fail the saving throw or will get hit.

Electrical Barrier. Zilliax starts with 50 temporary HP (can only stack form this source). Whenever Zilliax is subjected to lightning damage, it takes no damage and instead gains a number of temporary hit points equal to the lightning damage dealt as it is coated with an electrical barrier. While coated with an electrical barrier (has temporary hit points), all ranged attacks made against this creature are with disadvantage. It has a bonus +20 ft to its movement when Electrical Barrier is active. If a melee attack is made against this creature, they must make a Constitution Saving Throw DC 19, if failed, they take 13 (3d8) lightning damage.. If succeed they take half as much. Creatures that are wet, submerged, or wearing or wielding any metallic armor or weapon fails the saving throw automatically.

Immutable Form. Zilliax is immune to any spell or effect that would alter its form.

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

Magic Weapons. Zilliax's weapon attacks are magical.

Multiattack. Zilliax makes two melee attacks, two ranged attacks, or one of each.

Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 23 (3d10 + 7) slashing damage.

Lightning Ray. Ranged Weapon Attack: +13 {"diceNotation":"1d20+13","rollType":"to hit","rollAction":"Claw"} to hit,  100/200 ft., one target. Hit: 20 (3d8 + 7) lightning damage.

Discharge (Requires at least 1 temporary hit point) (Recharge 4-6). Zilliax can discharge his barrier as an explosive burst of electricity in a 30 ft radius around Zilliax losing all his barrier health. Each creature in that area must make a DC 19 Constitution saving throw, taking 45 (10d8) lightning damage on a failed save, or half as much damage on a successful one. Creatures that are wet, submerged, or wearing any metallic armor fails the saving throw automatically. 

Counterspell (5 Level). 2/day.

Zilliax 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. Zilliax regains spent legendary actions at the start of its turn.

Recharge. Zilliax gains 20 temporary hit points (Stackable with Electrical Barrier).

Claw. Zilliax makes a Claw attack.

Lightning Cannon (Costs 2 Actions). Zilliax fires a stroke of lightning from its claw forming a line 100 feet long and 5 feet wide blasts out from you in a direction you choose. Each creature in the line must make a Dexterity saving throw (DC 19). A creature takes (10d8) lightning damage on a failed save, or half as much damage on a successful one. The lightning ignites flammable objects in the area that aren’t being worn or carried. This recharges Zilliax’s Electrical Barrier, as Zilliax also takes the damage of this action.