Asar, the Eternal Serpent

Become Sand Asar has the ability once per day to sink into the sand and become a part of it the automatically hides Asar and gives him control of the sand around it for 1 round

Sand Swim Asar can dig and move through sand with ease. Ignoring difficult terrain in all desert regions. 

Siege Monster  Asar deals double damage to objects and structures.

Legendary Resistance (3/Day) If Asar fails a saving throw, he can choose to succeed instead.

Dreadful Glare Asar, The Eternal Serpent targets one creature it can see within 60 feet of it. If the target can see Asar, it must succeed on a DC 15 Wisdom saving throw against this magic or become frightened until the end of the Asar's next turn. If the target fails the saving throw by 5 or more, it is also paralyzed for the same duration. A target that succeeds on the saving throw is immune to the Dreadful Glare of all mummies (but Asar) for the next 24 hours

Putrid Fang Melee Weapon Attack: +10 to hit, reach 15 ft. one target. Hit: 45 (4d10+5) Piercing damage plus 18 (3d6) necrotic damage. If the target is a creature, it must succeed on a DC 16 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 18 (3d6) for every 24 hours that elapse. If the curse reduces the target's hit point maximum to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.

Sand Breath (Recharge 5-6). Asar expels a burst of sand in a 60 ft. cone. Each creature in that area must succeed on a DC 21 Dexterity saving throw, taking 63 (18d6) slashing damage and becoming blinded on a failed save, or half as much damage on a successful one. Creatures blinded by the attack may spend an action to remove the sand from their eyes and be freed of the blind condition. If a creature happens to drop to 0 Hp after being struck with Sand Breath, all the moisture will be drained from their body, turning them into a Mummy under the control of Asar. 

Swirling Sands Asar may use his reaction to cause an attack against him to suffer a -5 to the attack roll.

Asar 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. Asar regains spent legendary actions at the start of his turn.

Diving Slam Asar burrows for one turn taking no damage, then on his next turn he jumps up from the ground and slams down. 1 target Hit: 20 (1d10+5) bludgeoning damage.

Charging Swallow (Costs 2 Actions) Asar plows forward through the sand swallowing all in its path. 1 target Hit: 45 (4d12) The target, if hit, is then swallowed. They take 15 points of necrotic damage each round they remain swallowed. If Asar takes 25 damage in one turn he regurgitates every target he has swallowed. If Asar's hp hits 0, the target may exit Asar, moving 30 feet and lying prone. 

Sand prison (Costs 3 actions) Asar can control the sand under one creature it can see within 120ft. The sand encloses around the creature. The creature must make a DC 20 strength save or be restrained by the sand. Saves can be attempted at the beginning of each of the creatures turns as an action.