Serpent spawn of Thessalar

Amphibious. The serpent spawn can breathe air and water.

Otherworldly Perception. The serpent spawn can sense the presence of any creature within 30 feet of it that is invisible or on the Ethereal Plane. It can pinpoint such a creature that is moving.

Shapechanger. The serpent spawn can use its action to polymorph into a copy of any Small or Medium humanoid it has recently devoured, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies, is experiencing high emotionally distress or when it’s going to devour someone else.

Ambusher. The serpent spawn has advantage on attack rolls against any creature it has surprised.

Pack Tactics. The serpent spawn has advantage on an attack roll against a creature if at least one of the serpent spawn’s allies is within 5 feet of the creature and the ally isn't incapacitated.

Sunlight Sensitivity. While in sunlight, the serpent spawn has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Innate Spellcasting (Psionics).  Each individual serpent’s spawn innate spellcasting ability is Intelligence (spell save DC 17). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate and mage hand (the hand is invisible).

1/day each: confusion, dominate monster, eyebite, feeblemind, mass suggestion, plane shift (self only), project image, scrying, telekinesis, jump, misty step and nondetection (self only).

 Magic Resistance.

A serpent spawn  has advantage on saving throws against spells and other magical effects.

 Multiattack. A serpent spawn can use its Read thoughts or Serpentine embrace. It then makes six attacks: one with its bite, another one with its claws and a additional four melee attacks, each one with a different weapon.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage plus 5 (2d4) poison damage and the target is grappled (escape DC 13). Until this grapple ends, the target is restrained, and the serpent spawn can't bite another target. The target must succeed on a DC 11 Constitution saving throw or become paralysed for 3 hours by the serpent spawn ’s venom, until the start of its next turn.

Swallow. The serpent spawn  makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the serpent spawn, and it takes 10 (3d6) acid damage at the start of each of the serpent spawn's turns. The serpent spawn can have only one target swallowed at a time. If the serpent spawn  dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) slashing damage.

 Serpentine Embrace. The serpent spawn makes one claw attack against a Medium or smaller target it is grappling. If the attack hits, the target is constricted up in arms of the serpent spawn, and the grapple ends.The constricted  target is grappled and restrained, it is vulnerable against attacks and other actions their party make towards the serpent , and it takes 8 (4D6) crushing damage as the serpent spawn keeps squeezing them a little tighter at the start of each of it’s turns. The serpent spawn  can have only one target constricted at a time. If the serpent spawn loses it’s grip over them, a constricted creature is no longer restrained by it and can escape from the serpent spawn’s reach using 15 feet of movement, exiting prone.

Hand Crossbow of agonising slow death . Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 5 (1d6 + 2) piercing damage plus 6 (4d8 + 3) necrotic damage, and the target must succeed on a DC 18 Constitution saving throw or take additional necrotic damage for each failed saving throw. If the saving throw fails by 9 or more, the target will become unconscious and in slow long lasting agony while taking further necrotic damage. The target then can only be healed by a greater restoration spell or a wish spell.

Serpent Dagger of Thessalar. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage plus 7 (2d6) necrotic damage.

Cobra Scepter of Psionics. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 14 (4d6) psychic damage.

Read Thoughts. The serpent spawn magically reads the surface thoughts of one creature within 60 feet of it. The effect can penetrate barriers made from 3-6 feet of wood or dirt, 12 feet of stone, but 2 inches of common metals, or a thin sheet of lead blocks it. While the target is in range, the serpent spawn  can continue reading its thoughts, as long as the serpent spawn’s concentration isn't broken (as if concentrating on a spell). While reading the target's mind, the serpent spawn has advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.