OK, so I am putting some Glyphs of Warding on a trapped object in a dungeon, protected by a 7th level Witch. The Glyph triggers Vomit Swarm, which creates a 7-round long large swarm of wasps.
According to the Trap calculator, the CR of a Magical Trap is either based on the level of the spell, or how much damage it does, whichever gives you a higher CR. A Wasp Swarm does 2d6 damage (poison is irrelevant here) and lasts for 7 rounds, and is an area-effect. So 2d6 = 7HP per round, *7 rds = 49HP, rounded to the nearest 10 = 50, doubled for being AoE = 100HP/10HP blocks = 10, so CR = 1+10 = 11?
CR 11 for a trap that summons a CR3 creature? That does not seem right. Am I screwing up my math?
A trap that does 49 damage instantly could kill a low-level character. One that does 49 damage slowly, less if the party leaves or destroys the swarm, is obviously less dangerous.
I'd call it a CR3 trap.
Swarms are tricky. Very few folks have the resources to deal with them on a equal to CR rating...
Don't apply that guideline for creatures. That's for something like an acid spray that continues to burn for 7 rounds once you're hit. You can move out of the way or kill the swarm, so it's unlikely to actually deal that much damage.
Probably just treat it as the CR3 creature it is, since that accounts for the effects of being able to fight it.
That makes sense. thanks!