Got it - I thought I was probably forgetting something! I'm moving a bit to fast me thinks. I really haven't been taking that into account when I work up the CRs. I will have to look at all the swarms again.The swarm is resistant to all bludgeoning, slashing, and piercing... aka half damage from fighters. If the fighters get poisoned (which granted a 21 con save is not terrible for them), they would do half damage again from the poisoning (so 1/4 damage).
When I look at 4e swarms: they had half melee & ranged damage and vulnerability to area attacks. I only implemented half that (which is what is in the 5e MM). I get the resistance to BPS damage on swarms of tiny or small creatures, but maybe that should go away with larger creatures? So what do you think about these options for swarms / hordes of Medium and larger creatures:
- keep the resistance to BPS damage (current design)
- keep the resistance to BPS damage, but add vulnerability to areas of effect.
- drop BPS resistance and add vulnerability to areas of effect.
- drop BPS resistance and no other change