TEXT: Rules Variant: Dangerous Swarms
  C
  omments from the Thumb: It occurred to me the
  other day while building a dungeon that swarms
  aren't particularly threatening. Sure, maybe
  they're scary and annoying, especially to low level
  players, but let's look at the rules. We'll go with a
  swarm of insects for this example.
  Puny Bugs
  The swarm of insects has 22 HP and an AC of 12. While they
  are resistant to weapon damage and immune to just about
  every condition in the book, they're still no match for any one
  character over about level 2. Just a few swings of the
  barbarian's axe and they're mincemeat. The rogue can even
  sneak attack them! Imagine this: thousands of itty-bitty
  spiders are descending upon the party. The rogue steps up to
  bat. We'll assume they get sneak attack. A few slashes of a
  dagger and all 8436 (roughly estimated) spiders are dead.
  What. What even.
  A swarm should be easy to barbecue via fireball or the like,
  but nigh-impossible to melee to death unless you have a giant
  flyswatter, especially for swarms of insects. Have you ever
  tried to swat a fly out of the air? Try it with a sword. You have
  a few options here if you want your swarms to continue to be
  threatening at higher levels.
  Variant 1: More Threatening
  Swarms
  The first option is to beef up the swarm. Higher AC or HP is
  the simplest route. Maybe give them a trait that doubles their
  resistance, making attacks do 1/4 damage. For insect
  swarms, I might even make them outright immune to
  piercing and slashing. Definitely make them vulnerable to
  non-weapon damage to compensate though (fire, cold,
  lightning, acid, poison, force, thunder, etc.).
  Variant 2: Swarms as Hazards, Not
  Enemies
  Your next option is to ditch the stats altogether. The DMG
  contains rules for dungeon hazards. Do something like that.
  Here, I'll take a crack at it, once again using the swarm of
  insects. For fun, we'll use centipedes.
  Centipede Swarm
  A swarm of centipedes typically covers a 5 foot square, has
  10 hit points, an AC of 12, and is immune to nonmagical
  bludgeoning, slashing, and piercing damage. When a creature
  moves within 5 feet of the swarm for the first time or starts
  its turn there, the swarm immediately moves to occupy the
  creature's space, and the creature must make a DC 12
  Dexterity saving throw. On a successful save, the creature
  becomes aware of the swarm if it wasn't already, and suffers
  no further effects. On a failed save, the creature takes 1d6
  piercing damage as the centipedes begin to climb all over its
  body and bite it. The centipedes then move with the creature
  until they are removed or destroyed.
  At the start of the creature's next turn and every turn
  thereafter, it can use its action to make a DC 15 Dexterity
  saving throw. On a success, the creature shakes the
  centipedes off and the effect ends. On a failure, the creature
  takes another 1d6 piercing damage and 3d6 poison damage,
  and is poisoned for the next minute. A centipede swarm that
  is reduced to 0 hit points is instantly destroyed. The swarm
  can also be removed by jumping into water or similar
  methods, at the DM's discretion.
  One thing to note: as with everything in D&D, you can
  adjust all of the swarm's stats to make it harder or easier for
  your party. Feel like the saving throw DC is a bit low? Want
  the damage to be a bit higher? Maybe give it vulnerability to
  fire, cold, acid, etc.? That's your prerogative. You can also
  replace the centipedes with any other poisonous creepy
  crawly.
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/Mage Hand Press/Non-Published/Dangerous Swarms.pdf