Can I cast Slow on an invisible target?

Question

The following situation occurred yesterday during our session:

The sorcerer was stalking a guy he wanted to kill. They were in the middle of a city, so the sorcerer waited for an opportune moment to strike. Unexpectedly, the guy he followed cast invisibility on himself. The sorcerer didn’t want to lose him and decided to cast slow.

At first, I told the player of the sorcerer that he couldn’t do that, because his target is invisible now, even if he knew exactly where he was. Then we checked the slow spell and realized that the targets are chosen within a 40-foot cube.

Naturally, the player argued that line of sight isn’t necessary here, because the guy was the only one within the 40-foot cube, so he could just choose everybody within as target. I wasn’t sure if that’s correct. My counterargument was, that maybe there was an invisible school bus full of invisible children going down the street. The invisible school bus could be (without the sorcerer knowing) within the 40-foot cube. So there would be way too many targets within the 40-foot cube to target everything within it.

We discussed it, the players agreed and he found another way to locate and apprehend the guy.

Today I went back to the question and gave the slow spell a closer look.

It says “six creatures of your choice in a 40-foot cube”, not “six creatures of your choice you can see in a 40-foot cube”. I know that other spells have this “you can see” clause in their description. So I’m wondering how to handle this.

If there is an invisible evil dude and an invisible school bus full of invisible children all within the 40-foot cube, could the sorcerer choose any targets at all for the slow spell? Could he choose the evil dude specifically? Or would 6 random targets within the cube be slowed?

Answer 1

The Sorcerer can discriminate between invisible targets provided that they know the invisible targets are there

Once you are aware of an invisible creature, provided they are not hidden, you know where it is (because they are still making noise, kicking up dust, etc.) but you cannot, in general, see them.

If a spell specifies that you have to be able to see the target, then you have to be able to see the target.

Conversely, if it does not require you to be able to see them, then you can target them, so long as there is a clear line of sight/line of effect to the target [i.e. there isn’t a wall in the way].

The situations where you can see an invisible target are limited. Some examples of when you can see an invisible creature are:

• you have a lantern of revealing
• you have truesight
• you have blindsight
• some environmental effect, like heavy rain, is silhouetting the invisible creature

For the specific case of the Slow spell, since it does not have the “that you can see” qualifier, the Sorcerer can discriminate between targets that they know are within the area of effect.

So in your example, if they know the invisible BBEG is there and the BBEG is not hidden, then they can choose to only target the BBEG.

Answer 2

RAW: So long as they know the target is within the cube, then it can be done.

Beyond just the RAW: (Not so much RAI in-and-of itself, but you can see it in the same vein.)

However, let's say they were following a character who took levels in Rogue and has Cunning Action. Presuming you were treating things as though the characters were in Combat, regardless of if it was expressly stated or not, you could say it went like this:

1. Target walks down street.
2. PC is following on their "turn".
3. Target casts Invisibility on himself.
4. Target uses Cunning Action to take a Dash action to get away.
5. PC catches up to the area he last saw the Target in and casts Slow in a given area.
6. Target is outside the Area of Effect and is therefore unaffected by Slow.
7. PC wouldn't know if the spell worked or not and would have to decide how to proceed with this uncertainty. Depending on if the Target notices or not, a formal Combat order could begin.

This is just an example for showing the other side of the coin. In this case, the spell fails. In your scenario, the spell succeeds. If it was a matter of Slow specifying "that you can see", the Slow spell would then fail, but it doesn't. Essentially, all the Invisibility did was add another layer of potential failure to the Slow spell. Instead of the target simply having to succeed a Wisdom ST, now the caster has to be sure they are including their target in the area of effect or else it fails, and the PC won't know whether it failed because they missed or if their target succeeded the roll, which makes continuing their actions all the more difficult.

Answer 3

The spell description only states it affects an area (as you point out) and you can choose up to 6 targets within it. It doesn't specifically state that you have to be able to see the targets.

By contrast, other spells such as polymorph specifically state "a creature that you can see within range".

More generally, the rules say only that you must have a clear path to the target, so it can't be behind total cover. But again, visibility is not mentioned; only a spell requiring an attack roll would be affected due to rules about attacking a creature you can't see.

Answer 4

You've already received plenty of sufficient answers for your generic Can I cast Slow on an Invisible Target question, but you also ask a hypothetical about a school bus which hasn't received an answer.

You could not do this with your school bus hypothetical

The other answers focus on the need (or lack thereof) of line-of-sight. Some spells do, some spells don't, but the default answer is "does not require." This is not the case with line-of-effect, where "does require" is the default. In the scenario that prompted this question, the line-of-effect is clear: you could draw a straight line between you and your target without intersecting anything else, invisible or not. There are no obstacles, and there are no people in the way.

What would stop your Slow from hitting its target is full cover, which blocks line-of-effect. D&D has never provided a great rule about when you're in full cover and when you're not, but Jeremy Crawford has, somewhat controversially, said that glass (such as a school bus window) provides full cover. You could not cast Slow into a school bus:

A solid obstacle, regardless of material, can provide total cover. A closed window counts.

If you were talking about a school bus with, say, shattered windows, it's down to a DM call: what level of cover does this mass of innocent school children provide? There is no clear answer.