We had a little problem last session. Here's the scenario: there are enemies in the area of a darkness spell that the PC has never seen but has heard. He casts the spell slow on the area that includes the monsters.

How is this situation adjudicated? Are the creatures in the area of darkness (which the caster of slow can't see through) affected by the slow spell or not?

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Yes, the creatures are affected if the player aimed at the right spot regardless (e.g. if he knew exactly where they were because he heard them)

The Player heard them, aimed where he thought they were, so unless some Meta is involved which the DM could had handled differently, knowing their location is not required; luck could have been on his side too.

From the Spell Slow:

You alter time around up to six creatures of your choice in a 40-foot cube within range

It is not required for them to be seen if we read the description, since D&D has a lot of rules the spell cannot contain all the exceptions, but creatures he is not aware are there cannot be targeted (unless he tries an area randomly within the slow spell area).

The answers on this question can give you more details: Can I cast Slow on an invisible target?

Another proof this works (thanks V2Blast) is the Invalid Spell Targets optional rule in the Spellcasting section of the Xanathar's Guide to Everything (p. 85):

This section expands on the spellcasting rules presented in the Player's Handbook and the Dungeon Master's Guide, providing clarifications and new options.


Invalid Spell Targets

A spell specifies what a caster can target with it: any type of creature, a creature of a certain type (humanoid or beast, for instance), an object, an area, the caster, or something else. But what happens if a spell targets something that isn't a valid target? For example, someone might cast charm person on a creature believed to be a humanoid, not knowing that the target is in fact a vampire. If this issue comes up, handle it using the following rule.

If you cast a spell on someone or something that can't be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn't attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

So per this optional rule, you can target the area in a 5-foot square that you believe a creature would be and try at your own risk.

See the Question: Do you need line of sight to cast spells on someone? for more details. Especially Christian Griffith's Answer since it quotes Jeremy Crawford who is the official rules designer for 5e.

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