Giant Venomous Snake

A giant venomous snake on the party's side is almost always a druid's wild shape, a familiar handed up by a warlock with a serpentine patron, or the bound creature of a marsh witch the party has done a favor for. Commit to the framing in the first sentence of the scene, because nobody at the table will read "ally" on a CR 1/4 snake without wanting to know why.

Use the snake for a single specific job and then put it away. The 40-foot swim speed makes it a quiet courier across rivers, and Blindsight 10 ft. lets it function in pitch black if the party needs something retrieved from a flooded cellar. In a fight, the snake is a glass-cannon ambusher with 11 HP. Hold it back from the front line, send it up a wall to drop on a flanking enemy, then pull it out before someone hits it back. The Dex 18 means it goes early; spend that initiative on positioning, not on melee.

If the snake belongs to a wild-shaped druid, the player runs it and you only narrate. If the snake is the witch's gift, treat it as on loan: at the end of the day, or when the favor is repaid, the snake leaves through the nearest body of water without a goodbye. A snake is not a dog.

Let the snake survive the session. Players name things with eleven hit points, and the table will care more about the next swamp encounter for it.

A giant venomous snake is a swamp ambush, a sewer surprise, or a single bad step in long grass. The stat block is sparse on purpose: AC 14, 11 HP, Dex 18, 40-foot walk and swim, Blindsight 10. Run it as a one-round threat that defines the terrain rather than a fight that lasts more than two rounds.

Set up the surprise. Blindsight 10 ft. and a +2 Perception aren't great in the open, so the snake should already be in position when the party arrives, coiled in a reed bed or under a wagon. Roll Stealth or set a passive DC for the players to spot the strike: a Dex 18 snake going first on a 14 initiative is the entire encounter. The first PC into reach takes a hit; that PC is now the encounter's narrative center for the rest of the round.

The snake has 11 HP, which means almost any solid hit kills it. Plan for that. If you want the snake to actually do something, run two or three of them in flanking spots so the party doesn't get to delete the threat with a single Magic Missile. Spread them through the marsh: one in the water, one in the cattails, one above on a low branch if your version slithers up things. The 40-foot swim speed lets one disengage into the river and circle to bite the trailing PC again.

When a snake is below half, it tries to leave. Beasts don't fight to the death. Drop into the water, slip into the underbrush, and let the party either chase or move on. The party should remember the snake as the thing that made them roll a save before they could fix it, not the thing they killed for 50 XP.

Roll the snake's strike behind the screen and call out the result before naming the attacker. Players hate being surprised; that's what gives a CR 1/4 beast its weight.