Aeserpent

The aeserpent, also called darkstrike or deathstrike, is a giant snake up to 20 feet long. Its scales are pitch black with a glossy sheen that looks wet, even slimy, yet feel perfectly dry. In the dark only its narrow yellow eyes, and maybe its fangs, can be glimpsed. Many believe it is somehow vampiric, but it is neither undead nor magical, just a natural animal with a unique venom and superb camouflage.

Darkness is its weapon. In dark, near-dark or deep shadow it is effectively invisible and cannot be detected, and from ambush it imposes +2 on its foes' surprise rolls before biting.

A victim who fails a save vs. poison has their metabolism twisted: the body begins devouring its own blood, and death follows in 2d6 rounds of agony. Each point of Intelligence gives the victim a 5% chance to grasp what is happening, including that drinking other creatures' blood will prolong life, since the body now draws its oxygen, water and nourishment from blood. Slow poison and neutralize poison slow or halt the process but heal nothing, leaving the victim with just 1d12 hit points; cure light wounds, cure serious wounds and similar spells are needed to repair the damage.

The snake keeps biting after the first dose, and extra venom does little, but a natural 19 or 20 on a later attack means it swallows the poisoned victim alive. A swallowed victim is helpless unless already holding a short stabbing or slashing weapon (under 2 feet long), must otherwise be rescued from outside, and dies twice as fast as from poison alone. It never swallows on its first strike, as it cannot digest unpoisoned prey. Like all animals of the plane of beasts, it is immune to spells that affect animals, snake charm included.

Darkstrikes dwell mostly in the shadowed twilight layer of the plane of beasts, and also in its gray misty layer. They hunt alone except for occasional mated pairs, which are twice as deadly: they cooperate to herd prey into natural pits, dead ends, quicksand, landslides and similar traps, and gain +1 to attack and damage when defending a mate. A pair lays 1d8 eggs and guards them fiercely, but hatchlings fend for themselves.

Its digestion runs much faster than other snakes', so it must eat heavily every week, and in a normal ecosystem it would soon empty a region of prey. The plane of beasts' near-endless supply of animals, wolves and bears included, keeps it fed. One sage argues it came from the Abyss or another lower plane, but the creature is not evil, merely a hungry predator. If it came from elsewhere it was more likely a prime world, where it may have been hunted out of existence through fear, or it may simply have found the plane of beasts a better home.