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?