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 via the right subclass, or summoning via Conjure Animals), a yuan-ti contact who has lent the party her wards for one task, or a curse the party turned to their advantage. Pick the one that fits the campaign and stick with it for the scene.

In a fight the swarm's value is target denial in a confined space. The Medium swarm fills a 5-foot square and is Bludgeoning, Piercing, and Slashing resistant, which means it absorbs incoming 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 to use, and the enemy will spend a turn dealing with it before they can do anything to the party. The swim speed (30 ft.) makes this the same play in flooded environments where the party cannot easily form a line.

The condition immunity list is the second reason. Charmed, Frightened, Paralyzed, Petrified, Prone, Restrained, Stunned, and Grappled all bounce off. An enemy enchanter is wasting actions trying to lock the swarm down, and an enemy spellcaster who notices this and switches targets has just wasted a turn switching targets. The swarm is the perfect rude answer to a controller villain.

The swarm does not understand commands. Whoever summoned or directs it gives one short instruction (which space to occupy, which target to bite) and the swarm interprets it 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. The stat block lists no actions in this version, so the encounter rests on what the swarm IS: a Medium space full of small biting things, AC 14, 36 HP, swim speed 30 ft. equal to its land speed, and Blindsight 10 ft. The swarm does not need to see the party to find them, and it does not care which player it is biting first.

Use the resistance and immunity profile to teach the party what works against swarms. Bludgeoning, Piercing, and Slashing all halved means a longsword is a weak answer. The wizard's Hold Person does nothing because Paralyzed is on the immunity list. What works is area damage. A torch, a Burning Hands, a 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 a different encounter from a clearing where they are not. The Blindsight is short (10 ft.) but it means the swarm can attack in muddy water without disadvantage, which is when most swarms fall apart.

Swarms do not retreat. The Wisdom 10 and Beast intelligence (Int 1) means the snakes will stay on the prey until they are dead or scattered. Once you reduce the swarm below half HP, narrate the loss: snakes peeling off, dying, slithering away. Half HP is the point where the party should feel they are winning, even if the swarm fights to zero.

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.