Astral Serpent

Incorporeal Movement. The Astral Serpent can become incorporeal at will allowing it to move through other creatures and objects as if they were difficult terrain. It takes 15 (3d10) force damage if it ends its turn inside an object.

Inconsistant size. The Astral serpent has the ability to radically shift in size, while generally considered a large sized creature it can shift one size up or down during it’s turn as an action, with tiny being its minimum size and gargantuan being its maximum size. 

This affects it in a number of ways, for example when It increases in size it doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category each time the action is taken— from Medium to Large, or large to huge for example, with gargantuan being its maximum. If there isn't enough room for the target to double its size, the creature or object attains the maximum possible size in the space available. Each time the creature increases in size from Large, increase its attack rolls by 1d10, add +2 to its strength modifier and -1 to its dexterity modifier.

Conversely When reducing in size, the target's size is halved in all dimensions, and its weight is reduced to one-eighth it’s previous weightl. This reduction decreases its size by one category--from Medium to Small, or small to tiny for example, with tiny being the minimum. Each time the creature decreases in size, for example from large to medium, subtract its attack rolls by 1d10, add -1 to its strength modifier and +2 to its dexerity modifier. If the Astral Serpent has swallowed an unwilling creature, the Astral Serpent cannot reduce in size below one size larger than the swalllowed creature of the swallowed creature’s hit points are above 0.

Astral gate. The Astral Serpent has the ability to create Astral Gates that allow it to travel through the Astral Sea, this allows it to teleport from it’s current location to any desired location it can see within 300 feet, or to flee combat entirely by retreating into the Astral Sea. The Astral Serpent can bring along one grappled target and can bring along objects as long as their weight doesn't exceed what it can carry. It can also bring up to three willing creatures of its current size or smaller who are carrying gear up to their carrying capacity. The creatures must be within 10 feet of the Astral Serpent to be affected. 

If it would arrive in a place already occupied by an object or a creature, it and any creature traveling with it each take 6d6 force damage, and the action fails. 

Stomach hoard. The Astral Serpent has a unique second stomach that acts in a manner similar to a bag of holding, this allows it to store objects, nonliving creatures and material, or up to 3 willing creatures within its body without them being affected by its size changing ability. However it should be noted that breathing creatures inside the stomach can only survive up to a number of minutes equal to 60 divided by the number of creatures, after which time they begin to suffocate, and upon death are transferred to the primary stomach for digestion.

Multi-attack. The Astral Serpent makes three attacks these attacks can be any attack as long as the conditions are met excluding its disintegration breath.

Bite. Melee Weapon Attack: +10 to hit, reach 30 ft., one target. Hit: 31 (5d10 + 6) piercing damage.

Tail. Melee Weapon Attack: +9 to hit, reach 50 ft. one target. Hit: 20 (3d10+ 5) bludgeoning damage.

Constrict. Melee Weapon Attack: +10 to hit, reach 10 ft., one creature that is smaller than it. Hit: 22 (3d10 + 6) bludgeoning damage. The target is grappled (escape DC 17) if the Astral Serpent isn't already constricting a creature, and the target is restrained until this grapple ends.

Disintegration Breath (Recharge 5–6). The Astral Serpent exhales a line of force that is 60 feet long and 10 feet wide. Each creature in that line must make a DC 17 Dexterity saving throw, taking 80 (8d10 + 40) force damage. The target is disintegrated if this damage leaves it with 0 hit points A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell. This attack automatically disintegrates a Large or smaller nonmagical object or a creation of magical force. If the target is a Huge or larger object or creation of force, this attack disintegrates a 10-foot- sphere portion of it. Magic items are unaffected by this attack. This attack cannot be used as part of multi-attack 

Swallow. The Astral Serpent makes one bite attack against a Creature one or more sizes smaller than it, that it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the Astral Serpent, and it takes 40 (8d10) acid damage at the start of each of the Astral Serpent's turns. An Astral Serpent can swallow up to three creatures at a time dependent on it’s size, 1 huge creature, 2 large creatures, or three creatures of medium or smaller can be swallowed when the Astral Serpent is at its largest (gargantuan) size. If the Astral Serpent is smaller than its largest size then only one creature smaller than it can be swallowed at a time.

If the Astral Serpent takes 30 damage or more on a single turn from the swallowed creature, the Astral Serpent must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the Astral Serpent. If the AStral serpent dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Incorporeal Form. As a reaction the Astral Serpent can take an icorporeal form, the Astral Serpent must succeed on a DC 16 Dexterity saving throw, if successful the Astral Serpent becomes incorporeal and any attacks made against it are considered a miss.

The Astral Serpent 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 Astral Serpent regains spent legendary actions at the start of its turn

Tail Swipe. The Astral Serpent makes one tail attack.

Detect. The Astral Serpent makes a Wisdom (Perception) check.

Teleport (Costs 2 Actions). The Astral Serpent opens an Astral Gate in order to teleport, it along with any equipment it is wearing or carrying, or any target that is currently grappled teleport up to 300 feet to an unoccupied space it can see, or it can make a DC 14 wisdom saving throw to escape into the Astral sea.