Cerastalis

Skeletal weaponry. When the Cerastalis gathers bones, it gets a 5d6 damage bonus and 6d4 temporary hit points and the attack bonus goes away once their temporary HP drain to 0.

Nightmarish Mind. Any creature that reads the Cerastalis’ mind must make a DC 19 WIS save or take 3d20 psychic damage and becomes frightened.

Eternal Wrath. When slain the Cerastalis’ body explodes and all creatures within 195 ft dome must make a DC 20 DEX save or take 5d12 + 6 acid damage and on a success they take half. After being slain the Cerastalis’ skeleton will resurrect in one of the elemental planes and after 4d10 days it will fully reform and hunt down the last person who killed it.

Legendary resistance (9/day). If the Cerastalis fails a saving throw, it can choose to succeed instead.

Searing presence. When a creature enters within 5 ft of the Cerastalis must make a DC 16 CON save or take 1d8 fire damage.

Leathery Hide. When the Cerastalis is attacked by an attack that deals slashing or piercing, the attack is bounced off its hide and the attack is rerolled on 1-10 the attacker takes the damage instead.

Seige monster. The Cerastalis deals double damage to objects and structures.

Sun-like illumination. The Cerastalis produces a bright light up to 60 ft away from it, and produces a dim light up to an extra 120 ft away, the light the Cerastalis produces is considered Sun-light, and thus as the same effects as sun-light. If the Cerastalis is in magical darkness, it destroys the magical darkness. This light also prevents lycanthropes from transforming in it.

Macro-targeter. If the Cerastalis makes a weapon attack against a creature of small or tiny size, the attack has disadvantage. If the Cerastalis makes a weapon attack against a creature of large, huge, or gargantuan size has advantage. If the Cerastalis makes a weapon attack against a gargantuan creature, if the attack hits its instantly a critical hit.

Amphibious. The Cerastalis can breath air and water.

Camouflage. The Cerastalis has advantage on all stealth rolls and gives disadvantage to all creature’s perception rolls against it while it’s camouflaged. While being stealthy the Cerastalis can use a bonus action to become invisible. 

Swallow. The Cerastalis makes a bite attack against a large or smaller creature it is grappling, if the bite attack lands the target takes the bite damage, is swallowed, the grapple ends, and the creature while being swallowed is restrained, blinded, and deafened. The creature is immune to all damage and effects outside the Cerastalis, and the creature takes 32d6 acid damage at the end of the round. If the Cerastalis takes 60 damage from one attack it must make a DC 18 CON save or regurgitate the creature which fall prone within a space ten feet away from the Cerastalis. If the Cerastalis dies the he swallowed creature is instantly killed.

Acidic ray(4-6 recharge). The Cerastalis fires a 80 ft long and 21 ft wide. Each creature in that cone must make a DC 21 DEX save, on a fail, they take 5d12 + 23 acid damage and they’re flung 20 ft away and on a success they take Thales as much damage and aren’t flung.

Multiattack. The Cerastalis can make 1 bite/swallow, 2 claw slashes, 1 acidic tail skewer, and 4 psionic darts.

Bite. Melee Weapon Attack: +7 to hit, reach 20 ft., 1 target. Hit: 3d12 + 12 piercing damage and target makes a DC 19 STR save, on a fail the target becomes grappled.

Claw slash. Melee Weapon Attack: +7 to hit, reach 40 ft., 1 target. Hit: 4d12 + 7 slash damage and the Cerastalis must make a DC 21 DEX save, on a fail the Cerastalis is launched 15 ft away from where the Cerastalis was located and is knocked prone, on a success the Cerastalis is launched 20 ft toward the target and gets a bonus 20 ft of movement.

Acidic tail skewer. Melee Weapon Attack: +9 to hit, reach 120 ft., 1 target. Hit: 12d6 + 5 piercing damage and 3d12 acid damage, and the target must make a DC 16 CON save, on a fail the target becomes poisoned.

Psionic darts. RangedWeapon Attack: +9 to hit, range 9/120 ft., 2 target. Hit: 4d8 + 5 piercing damage and the target must make a DC 18 INT save, on a fail the target takes 5d12 + 9 psychic damage and they become frightened.

Bonus actions have to come before any other actions minus legendary, mythic, or reactions.

Psionic guard. Temporarily reduces all damage taken from spells and temporarily becomes immune to psychic damage. 
Haunting bone flame. For 2d6 rounds, all physical attacks against the cerastalis are reduced to one quarter of the original damage and the attacker takes 2d8 fire damage.

The Cerastalis can make 5 legendary actions a round and can be used during an ally’s turn.

Tentacle lash (Costs 2 Actions). Melee Weapon Attack: +9 to hit, reach 60 ft., 1 target. Hit: 6d12 + 19 bludgeoning  damage and the target is knocked prone.

Stopping gaze (Costs 3 Actions). The Cerastalis forces a creature it can see to make a DC 20 WIS save, on a fail the target can’t make any more than one type of action a round.

Acidic Roar (Costs 5 Actions). The Cerastalis emits a roar, roar is a 120 ft long 40 ft wide cone. All potential targets in the cone must make a DC 20 CON save, on a fail the creature takes 6d12 + 12 acid + 1d12 thunder damage, then the targets that fail the CON save must make a DC 19 CON save, on a fail the target becomes deafened and takes an additional 2d10 thunder damage.

The Cerstalis can make 4 mythic actions and can be used as attacks of opportunity and ready actions. 

Hellish black wave. All creatures in a 60 ft sphere around the cerastalis must succeed a DC 18 CON save or take 5d12 + 6 necrotic damage and become unconscious.

Call Allies (Costs 2 Actions). Roll a 1d6, depending on the roll summon these creatures (1-3) 2d10 lightning wisps, (4-5) 2d6 Draptils, or (6) 1d4 Gatorjaws

Acidic jet (Costs 4 Actions). Waits for two rounds then unleashes a 120 ft line in front  of it, all creatures in that line must make a DC 21 DEX save or take 12d12 + 19 acid damage