Can warlocks choose who is affected by fey presence, or does it automatically affect everyone in the 10-ft cube? I assume it to be like Presence in Vampire the Masquerade. The wording is:
"…As an action, you can cause each creature in a 10-ft cube (..) to make a Wisdom check…"
Now I read this as: as an action, you can cause each (all) creature to make a wisdom check. As in, For an action, you can either make them all take a check, or none of them. Kind of like you radiate this presence, and anyone near you is scared or charmed.
The player feels as though he should be able to target specific people. I realize it is up to the DM, but I don't want to screw the player. In fact, I am thinking about just letting it go and allowing him to choose just so he has more fun with it, but I want to know what you guys think.
Best Answer
It's every creature...
Area of Effect spells and abilities, unless specified otherwise, hit everyone in their area. The same wording "each creature in a [certain] foot radius" is present in Fireball. By the logic of your fellow player, you should also be able to decide which creatures are affected by fireball. But that is not the case.
For reference, a spell that does let you choose creatures inside of an area is Slow, which has a 20 foot cube and allows you to choose 6 creatures. So, because the phrase "creatures you choose" is not present in the description of Fey Presence, it affects everyone in the specified area.
The use of the word can is not relevant here, since it is stating that you can use an action to use this ability, not that you can or cannot affect certain creatures.
But...
you can choose whether they are charmed or frightened by you, and a player charmed by another player in combat may not behave any differently than before, although they might not particularly care to be charmed by you. It only lasts until the end of your next turn.
Caveat
There is no hard and fast rule in the PHB that says all AoE spells behave thusly. However, the description of each spell will tell you whether or not you can pick and choose targets. If you can't target creatures individually in the area, it always says "each creature", as opposed to "each creature you choose" or "up to X creatures you choose". You need to look at each spell individually to see how it behaves.