Corphet, The Venomwing

Semi-Immortal Nature. Corphet can't die of old age unless affected by magic of a spell of 6th level or higher and doesn't require food, drink, air, or sleep.

Sand-Slitherer. Corphet can move across and climb stony, glassy, and sandy surfaces without making an ability check. Additionally, difficult terrain composed of sand and/or stone doesn't cost him extra movement.

Petrifying Throes. When Corphet dies, his body rapidly transforms into loose sandstone that disintegrates. This destroys all equipment he was wearing or carrying and all equipment that weighs less than 3 lbs that were attached to his body.

Multiattack. Corphet makes two melee attacks and one ranged attack.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 7 (1d8 + 3) piercing damage, and the target must make a DC 17 Constitution saving throw, taking 17 (5d6) poison damage on a failed save, or half as much damage on a successful one.

Tail. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 12 (2d8 + 3) bludgeoning damage. Corphet can choose to instead deal half as much damage and grapple the target (escape DC 16) if it is a Huge or smaller creature. While grappled in this way, the creature is restrained, and Corphet can't use its Tail to make attacks.

Venom Spit. Ranged Weapon Attack: +9 to hit, range 90 ft., one target. Hit: 9 (1d10 + 4) poison damage + 5 (1d10) acid damage.

Acidic Barrage (Recharge 4-6). Corphet makes three ranged attacks, each with advantage.

Acid Splurt. Corphet belches out a giant glob of acid into the air that bursts into 1d4+1 smaller blobs that land at random points, each within 25 feet of a point within 80 feet of Corphet. When the blobs land, each target within 10 feet of the landing point must make a DC 16 Dexterity saving throw, taking 14 (4d6) acid damage on a failed save, or half as much on a success. A target that gets caught in the area of multiple blobs After landing, for 1 minute, the blobs of acid become 10-foot wide pools, and whenever a creature starts its turn while in the area of a pool it takes 5 (1d10) acid damage. 

Venomous Breath (Recharge 5–6). Corophet exhales acidic venom in a 45-foot cone. Each creature in that area must make a DC 19 Dexterity saving throw, taking 27 (6d8) poison damage + 18 (4d8) acid damage on a failed save, or half as much damage on a successful one.

Corphet can take 2 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. Corphet regains spent legendary actions at the start of his turn.

Sense. Corphet makes a Wisdom (Perception or Nature) check.

Attack. Corphet makes a melee attack.

Hollowed of The Sands (Costs 2 Actions). Corphet summons 1d6+1 mummies with resistance to acid damage that appear at points Corphet can see within 50 feet of him. The mummies last for 5 minutes until either they or Corphet dies, they serve him as allies, and take their turns immediately after him.