Can you cast Insect Plague into another room with and/or without scrying? [duplicate]

Question

So there's a potential scenario coming up in my game. I've "accidentally" given my players a very fortifiable position that could make a cakewalk of a rather grueling fight if they think to use it. It's a small jail with a heavy, sturdy door they could block themselves in with. I have this idea that one of the enemy spellcasters could cast Insect Plague into their room, since he's familiar with that area and the spell doesn't have to be a "point you can see" but instead a "point you choose within range". I'd use this as a tool to force them out of this location and hopefully provide a more interesting fight.

Since it's a conjuration spell, I imagine that it summons the insects FROM that point, so the rule on page 204 of the PHB that says "you need a clear path to your target" doesn't seem like it would apply... Would it?

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

The wording of that makes me think that the wall would block the spell. If that's the case, this caster also has access to a magic item that gives him the ability to cast the Scrying spell. Since he'd know they're in this jail, he could spawn the sensor inside, then have line of sight to cast Insect Plague.

There's a problem here, though, because Scrying and Insect Plague both require concentration. Would it still be possible for him to cast Insect Plague, but just immediately break concentration on scrying, or would this be impossible?

Answer 1

Line of effect is always required

The "point you choose within range" is the target of the conjuration spell, which means the normal rules for targeting apply, including the requirement for line of effect, as you have correctly surmised. Certain spells, such as message, explicitly bypass the need for direct line of effect and provide an alternative criterion for whether they can reach their target. In the absence of such text, a spell always obeys the normal rules for line of effect.

Even if you have some means to see the other side of a wall, you still have to cast the spell from your current location, which means you need line of effect from your current location. Hence, it is possible to have line of sight on a target without having line of effect. (You don't even need a divination spell for this; consider looking at a target through a glass window.)

Concentration is lost as soon as you start casting another concentration spell

This aspect is adequately covered by another question on this site: Can a concentration spell be cast without actually concentrating on it for an "instant" effect?

In short, you lose concentration as soon as you start casting another concentration spell, so even if it were possible to cast spells "through" your scrying sensor, you would not be able to do so with spells that require concentration, because the scrying sensor would be gone by the time you finished casting.