As a question that came up from this discussion about whether Mind Blank foils all attempts to gain information about the target, can area of effect spells be used to reveal enemies?
In the example of a fireball spell. Fireball does not have to target an enemy directly; the caster can specify a specific location, and the fireball effects the entire area within the spell's radius.
But would that fireball reveal to the caster any hidden or invisible enemies or things in that area?
Best Answer
It depends on what the enemy does in reaction to being hit by fireball, which is situational and up to the GM
The fireball spell description doesn't say that it reveals enemies, but on the other hand, it doesn't preclude that immediately subsequent events would reveal them.
For example, the GM might deem that a hidden or invisible enemy would cry out in pain, thereby giving an auditory indication of their presence (but perhaps not pinpointing their exact location -- unless you win on a passive Perception check). But alternatively, the GM might deem that the enemy has the toughness of will to remain silent.
Also consider that even if fireball "misses" (does zero damage, e.g. if a rogue prevails in using his evasion feature), a similar thing could happen. Doing zero damage to a hidden and evasive rogue in the area requires them to have made a Dex save, which implies that the enemy jumped or dove out of the way in the nick of time. The GM might deem that a hidden creature diving away from a fireball, is thus visibly revealed. Or the GM might deem that the hidden creature has a large enough hiding space in which to jump away and still remain hidden.
As another example, suppose an NPC caster is making herself invisible by casting a spell; she might lose concentration as a result of taking damage from fireball, and thus become visible. But of course, she might not lose concentration, and remain unseen. So again, the outcome cannot be known until we see what happens in the next instant after the fireball's explosion.
Nothing in the spell's description entitles us to infer that enemies are automatically revealed (along the lines of the accepted answer to this question), but in some situations, the GM could reasonably determine that what enemies do in the situation does reveal their general presence, or even which space they are in. And likewise, I would think, for most any AoE spell effect.