Because we (that is some of us) want more summons, and also because I can, I've adapted the old 2e summon swarm spell to 5e. Is it balanced? I threw it together using other conjure X spells as a guide, I'm thinking that doubling the base number of swarms might make it more worthwhile but then it is only a 2nd level spell slot so I dunno.
Summon Swarm
2nd-level conjuration
Casting Time: 1 Action
Range: 60 feet
Components: V, S, M (a square of red cloth)
Duration: Concentration, up to 1 minute
You summon up one or more swarms (See MM p. 337-339) of small animals in unoccupied spaces that you can see within range. Choose one of the following options for what appears:
- One swarm of challenge rating ½ or lower
- Two swarms of challenge rating ¼ or lower
The swarms will attack any creature in range as chosen by the caster. Roll initiative for the summoned swarms as a group, which has its own turns. They obey any verbal commands that you issue to them (no action required by you). If you don’t issue any commands to them, they defend themselves from hostile creatures but otherwise take no actions.
If the caster does not remain within 60 feet of a summoned swarm then it disperses harmlessly.
At Higher Levels. When you cast this spell using certain higher level spell slots, you choose one of the summoning options above, and more swarms appear: twice as many with a 4th-level slot, three times as many with a 6th-level slot, and four times as many with an 8th-level slot.