Challenge Rating Conditions. This creature's challenge rating depends on the character level of the summoner or tamer. It's Creation is mostly due to the summoning by a mage with the

Spellcasting. The Abyssal Serpent is a 3rd-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 13, +3 to hit with spell attacks). The Abyssal Serpent has the following Wizard/Warlock spells prepared:

• Cantrips (at will): Lightning Lure, Toll The Dead, Eldritch Blast, and Chill Touch
• 1st level (4 slots): Ice Knife, Fog Cloud,  Armor of Agathys, Leviathan's Grasp
• 2nd level (2 slots): Mirror Image, Ray of Enfeeblement

Aquatic Prowess (Water only). When submerged in any liquid terrain, the beast deals an extra 1d4 to attack rolls and has an advantage on attack rolls and saving throws.

Evasive Sub-Mariner (Water only). When submerged in water the aquatic beast never provides an opportunity to attack but can always make one irrespective of the disengage action. Whenever it takes the attack action it halves the speed of the target. It also suffers a disadvantage on Dexterity saving throws in dry terrain.

Multiattack. The beast makes several attacks equal to half of its proficiency bonus (rounded down)

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 5 (1d4 + 3) piercing damage. 

Scale Shot. Ranged Weapon Attack: +3 to hit, range 15/20 ft., 2 target. Hit: 4 (1d4+ 2) piercing damage.

Coiled Defense. The Abyssal Serpent when targeted by any melee attacks can curl up its body in an attempt to overlap its scales increasing its durability, on a hit it gains +3 to AC.

Abyssal Strike: (Costs 1 Action, but can only be used three times per summon)
The Abyssal Serpent can use its action to make a constricting attack on an enemy within 10 feet, dealing 2d6 + 2 bludgeoning damage and restraining the target (escape Strength Save DC:14). While restrained, the target takes 1d6 cold damage at the start of each of its turns.