Here's a rules question about spells that one can preserve in being with concentration, potentially for a very long time. Do you have to be able to see them the whole time to concentrate on them? The specific case I have in mind is as follows: I have summon swarm prepared, and the party is about to enter a new room we're confident has baddies in it. We're pretty sure they don't know we're there yet. I'd like to start casting the spell (full round), have an ally open the door as the casting is completed, summon the swarm into the room, and close the door. Could I keep the swarm in being with closed door between us so long as I'm concentrating, or not? Does "duration: concentration" really mean "concentration and observation"? I haven't been able to locate anything in the RAW to decide it. And I imagine the issue also applies to other spells like illusions that can stick around as long as the caster concentrates. What do you think?
I'm not aware of anything that requires LOS to concentrate on a spell.
Lack of LoS, depending on spell, may prevent you from manipulating the spell in a meaningful manner. Spell range may also be a factor.
For example the spell flaming sphere (while not a duration concentration spell) has the following: "A flaming sphere winks out if it exceeds the spell's range."
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RAW seems to indicate that LOS (and line of effect) are only required to cast a spell or create an effect, not to maintain it.
I am not sure if that is the intent.
Lack of LoS, depending on spell, may prevent you from manipulating the spell in a meaningful manner.
Example: directing your illusion to react appropriately to being attacked such that it isn't automatically disbelieved.
Also going to have a problem moving any spells that need to be directed to new targets (like flaming sphere) though I can't think of any of those with duration (concentration), and summon swarm specifically follows targets on its own and can't be directed, so no problem there.
Yeah, I have the same read on the rules that you both do, so that's comforting. And there's also no problem with the summoned swarm moving out of the spell's original casting range once they're present, too, as is clearly stated in the description of conjuration spells in general. I guess what I'm left wondering is, what exactly is the caster concentrating *on*? He can't control the swarm's activity, so is he just preserving their presence on his plane by sheer will power, whether he has line of sight or line of effect to them or not?
Anyway, it's maybe going to be an issue when I try this in a session tonight, so hopefully my DM will see things this way too.
And another thought: what if the thing keeping you from observing the swarm wasn't a door, but a fog cloud? Surely we wouldn't say that losing sight of the swarm in a fog meant you couldn't concentrate on keeping it around. That counts against line of sight being an issue. And line of effect doesn't seem to be an issue either, for this spell anyway, or at least that's my intuition.