Slithermorph

Most of the time a slithermorph looks like a black pudding, a slick, glistening black blob creeping along. At will it can take a second shape, a heavy-bodied scaled serpent with four muscular arms near the head, much like a vasshari; that 'fighting form' uses the parenthetical statistics. Both shapes are amphibious.

Each form has 90-foot infravision and is immune to poison, cold, fire and acid, but lightning and magic affect it normally. Shifting takes a round in which it cannot attack, and it may switch as often as it likes. Once per day, a shift heals 25% of the damage it has taken, and it also regenerates 3 hit points per day.

As an ooze it squeezes through gaps an inch wide or larger and creeps over walls and ceilings at full speed, often dropping onto prey, wrapping it up and squeezing for 1d6 per round. In serpent form each arm claws for 3-12 and can grab and pin a target. The talons dig through eight inches of wood or loose earth in a round, though stone and metal resist them. It can wield clubs and similar crude weapons at +1 to hit and +3 damage for its 18/49 Strength.

In either form it can release acid from any part of its body four times a day, usually where it is wrapped around a victim. The acid inflicts 3d4 damage (2d4 underwater), halved on a successful save vs. petrification, and forces item saves. It eats an inch of wood per round and readily corrodes metal: chain mail in two rounds, while plate armor, helms, shields and similar solid pieces last four. Each plus of enchantment adds a round of resistance, so a shield +4 lasts 8 rounds. Any metal item may save to escape entirely, needing 13 or better, +1 per plus and +1 for pure or alloyed adamantite, mithril or silver (electrum and similar metals at the DM's option).

They rest on high ledges, stalactite-studded ceilings, thickets or underwater weed beds. They infest many underground rivers, including a great subterranean river that lets them roam the lower reaches of deep dungeon complexes and the vast caverns beyond. Solitary, they mate whenever male meets female and fight only when two females meet one male, battling until one dies or flees. Six months later the mother bears one live, half-strength offspring that reaches full size and power in two to five months.

Each slithermorph roams far to claim a hunting ground away from others of its kind, then tends it like a rancher, taming animals, removing predators and harmful litter, and planting food for its 'herds,' which may include clams, fish, crayfish, worms, spiders or any mix.