Shadow Walker:
this creature walks within the shadows of mortal beings. residing in an otherwise inky darkness that it calls home. when hosted with another creature. it will live within and take on the a size more comparable to its host. it make make any of its melee of ranged attacks from this hosts shadow.  lunging out for a moment before retreating within. 

Serpents Strength: Innate
while connected to a host the serpent may make 2 actions per turn.
Serpents Bite: 1 action
make a melee attack against a target within range. on a successful attack deal 3d6 piercing damage and 1d6 necrotic. if the target is off a smaller size then the serpent, then they may use their bonus action to attempt to swallow the target.

Coiling strike: 1 action
When a target within melee range has failed an attack roll against the Abyssal serpent, it may make an attack against the target to coil around and contract them. the creature must make a STR saving throw or become grappled. taking 3d8 bludgeoning damage per turn it remains constricted.

Venom Shot: 1 action
rearing back its head The serpent will spit its corrosive venom at a target within 60ft. they must make a DEX saving throw or become Poisoned. taking 2d4 necrotic damage per turn.

Enraged Strike: 1 action
When a foe within 40ft makes a successful strike against the serpent, and it has an action available. it may rush the target head first bashing into it and driving it into the nearest structure. if there are no structures within range it will simply knock the target back 15 ft. the target must succeed a DEX saving throw to avoid the serpent, or take 4d4 bludgeoning damage. 

Swallow: 1 bonus action
when a creature has been successfully bitten, the serpent of the abyss may attempt to consume its target. they must make a STR saving throw or be swallowed. a creature may attempt each turn to escape the serpents stomach. on a failed save they will take 2d4 necrotic damage.

Petrify: 2 Bonus actions
when a target first meets the Serpent of the Abyss, it may attempt to overpower them with fear. the target must succeed a Wisdom saving throw DC 10+ CHA modifier or become feared until a successful save. while in this state the target cannot move, make an attack action or  speak.