Swarm of Venomous Snakes

A swarm of venomous snakes on the party's side is a stretch worth taking. The framing is almost always a druid (wild shape into a swarm, or summoning via Conjure Animals), a yuan-ti contact who lends the party her wards for one task, or a curse the party turned to their advantage.

In a fight the swarm's value is target denial in confined space. The Medium swarm fills a 5-foot square and is bludgeoning, piercing, and slashing resistant, absorbing weapon attacks the party would otherwise take. Park it in a doorway, on top of a downed PC, or in the corridor the enemy needs, and the enemy spends a turn dealing with it before doing anything else. Swim 30 ft. makes this the same play in flooded environments where the party cannot form a line.

Condition immunities to charmed, frightened, paralyzed, petrified, prone, restrained, stunned, and grappled make the swarm the perfect rude answer to a controller villain. An enemy enchanter wastes actions trying to lock the swarm down. An enemy spellcaster who notices and switches targets has just wasted a turn.

The swarm does not understand commands. Whoever summons or directs it gives one short instruction (which space to occupy, which target to bite) and the swarm interprets loosely. Treat it like a wild force the party is pointing, not a unit they are commanding.

Have the swarm leave a single snake behind when it dissipates, curled up somewhere strange. The party will spend the next session arguing over whether to feed it.

A swarm of venomous snakes is the encounter that happens when the party walks into the wrong patch of grass, opens the wrong sarcophagus, or wades through the wrong piece of swamp. AC 14, 36 HP, walk 30 ft., swim 30 ft., Blindsight 10 ft. The Bites action deals 1d8+4 piercing damage (or 1d4+4 if bloodied) plus 3d6 poison damage. The swarm does not need to see the party to find them, and it does not care which player it bites first.

Use the resistance and immunity profile to teach the party what works against swarms. Bludgeoning, piercing, and slashing are halved, so a longsword is a weak answer. Hold Person does nothing because Paralyzed is on the immunity list. What works is area damage: a torch, Burning Hands, Thunderwave, a flask of oil. Make the rogue with the dagger feel useless for one round so that the answer feels earned.

The swim speed is the part of the stat block most GMs forget. Drop the swarm in shallow water and let it close at full speed across terrain that slows the party. A river crossing where the snakes are already in the water is different from a clearing. Blindsight 10 ft. means the swarm attacks in muddy water without disadvantage, which is when most swarms fall apart.

Swarms do not retreat. Intelligence 1 and unaligned disposition mean the snakes stay on the prey until dead or scattered. Once you reduce the swarm below 18 HP, narrate the loss: snakes peeling off, dying, slithering away. Half HP is when the party should feel they are winning.

Set the encounter somewhere the players will be unwilling to use fire. A library, a wooden ship, a hayloft. Forcing the wizard to choose between Burning Hands and the next plot hook makes the encounter mean something.