Given the long march of ages, and a steady diet of cave-dwelling creatures and adventurers, and the absence of the gods' love, sometimes an arachnophidian grows to elephantine proportions. At such a size, it begins to resemble a hydra, and though it lacks the latter's regenerative abilities it makes up for it with serpentine coordination.
Slumbering Ancient. Thanks to its cold blood and ponderous metabolism, a spydra can lie in wait for months at a time until prey stumbles upon it. If female, the spydra may hibernate near a colony of her eggs, awakening to devour her own young if the year grows lean.
Waterborne Webs. With sixteen nostrils and a heavy abdomen, a spydra often makes its home near underwater streams and pools where it can lurk with ease. It fills the waters of its lair with a webbing forest-- thick, globby strands of silk easily mistaken for an aquatic plant (DC 15 survival or investigation to identify, DC 15 Dexterity save on contact or become restrained until the beginning of your next turn). It feeds on whatever the stream brings it, filtering out the large prey from the small.
The Spydra's eight legs function much like additional creatures in combat. Fort the purposes of calculating encounter difficulty, consider the whole Spydra to be two CR 8 monsters.