Abyssal Potential. These monsters, when summoned as an ally of the summoner and not in a subjugation ritual, their hitdie will increase by an amount equal to (Caster Level - 4), and their damage die will increase by an amount equal to (amount of Ability Score Improvements - 1) they've had.
Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage.
Constrict. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 13 (2d8 + 4) bludgeoning damage, and the target is grappled (escape DC 13). Until this grapple ends, the creature is restrained, and Great Serpent can't constrict another target.