Constrictor Snake

A constrictor snake on the party's side is a familiar, a druid's wild shape, or a serpent-cult cleric's gift. Snakes are Int 1 and don't bond, so any "ally" frame is really a frame for the handler. Decide who's holding the leash before the scene starts.

The snake is best used out of combat. 30 ft. swim plus Stealth +4 makes it the cheapest underwater scout in the game. It can investigate a wreck, slip into a flooded cellar, or carry a small object across a moat. Blindsight 10 ft. lets it work in mud or dark water where a human scout would need a torch. In overland travel, a draped snake on the druid's shoulders provides a Perception +2 watch shift that doesn't need to sleep on a human schedule.

In combat the snake is a one-trick distraction. Send it onto an enemy spellcaster's turn, force the caster to deal with a Large beast suddenly in their face, and accept that the snake will be dead by the end of the round. With 13 HP and AC 13 it cannot tank, but it can buy one missed Concentration check or one wasted enemy attack, which at low levels can decide a fight.

The snake will not refuse an order, but it also won't improvise. If the handler is dropped, Charmed, or out of line of sight, the snake goes still and waits. It does not avenge. It does not retreat to safety. It just stops being useful, which is its own kind of liability the party should plan around.

A constrictor snake is an ambush, not a fight. 13 HP and AC 13 means it dies to one good round of focused attacks, so the entire encounter has to happen in the surprise window or it doesn't happen at all. Build the scene around concealment: tall grass, hanging vines, a flooded cellar with the snake half-submerged, a temple chamber with a snake-shaped stone column that turns out to be warm.

Stealth is +4 and the snake has 30 ft. walk plus 30 ft. swim, so it can hide in either water or undergrowth and close on whoever has the lowest Passive Perception. Blindsight 10 ft. lets it operate in pitch dark or murky water without disadvantage as long as the target is close. Pick one party member, usually the trailing scout or the wizard at the back of the marching order, and have the snake close the gap before initiative gets called. That's the one moment of fear the snake provides.

In open combat the snake is outmatched. Int 1 means no tactics, no flanking, no second target. It bites the closest threat and hangs on. With 13 HP it's gone after one focused turn, so let the players have the catharsis of dropping it cleanly. The snake's job in the encounter is to bloody one PC, not to win a fight against the party.

If the snake survives long enough to be bloodied (around 6 HP) and the prey isn't already wrapped up, it disengages on instinct and slithers into water or undergrowth. Snakes are not committed. They are hungry, and a hungry snake gives up on prey that hits back too hard.

Describe the strike before you call for initiative. The grass shifts, something cool brushes a calf, and then the wizard is on the ground. That's the encounter.