Minor Illusion allows the creation of a 5-foot illusory object: can I hide behind this object (wall/crate/anything) and gain advantage vs. creatures who can't see past the illusion?
If so, given that examining the illusion requires an action, am I guaranteed to have advantage at least until their turn arrives?
Can you hide behind the illusion? Probably. Can you get advantage just for being behind it? Probably not unless you're a halfling.
Here's the problem, ultimately, you're only creating a 5' cube. That's probably not enough to hide behind without an actual stealth check.
Basically, you need to completely block line of sight from the creature you want advantage on, to yourself. You also, theoretically need to maintain said blockage of line of sight to your opponent until the moment you attack...
The way this probably works best is if you're a halfling and cast this the turn before. Then you could duck back, and shoot through the illusion with advantage.
For a normal sized character, without an action to hide, I'd probably not allow you to remove yourself from LOS behind the illusion (that's not to say that it doesn't block). Most characters are over 5' tall (a huge swath of humans are, like probably 80% or more, and likely even more adventuring humans). That means that even though you only take up a 5' cube in game space, you'd be seen over an illusory one. You'd get cover...but the thing is an illusion so if you're shot at, I'd give the creature the impression you had cover, but none would be applied to the attack (or maybe half the cover number, so -1 or -2 instead of -2/-5). Advantage on you attacks though, not so much.
However, if you're willing to spend the action (to make a dex(stealth) check) to hide, I'd acknowledge that, because you're actually spending a whole action, and that is modeled by being quiet, stepping softly, and possibly crouching behind an illusory object.
So yes, the wall is opaque, and you'd be able to hide behind it/have advantage from attacks (technically, if it was a real object, they wouldn't be able to target you..I'd allow the targeting)...but only if you completely fit behind it, and that's not the case for most PCs.
"If the creature discerns the illusion for what it is, the illusion becomes faint to the creature." - so yes, you can see through your own cover.
"it must be no larger than a 5-foot cube" - on a grid, medium characters take up a 5' cube (even if they are technically 6' tall). The size table in the PHB says "Medium: 5 by 5 ft."
"A target has total cover if it is completely concealed by an obstacle." If your illusion is say a solid 5' cube of steel, you will be completely concealed in its square.
You do not need to hide. You have advantage merely as a result of being an unseen attacker. "When a creature can’t see you, you have advantage on attack rolls against it."
In short, this works just fine.
To avoid arguments with your DM however, you may want to just make this a 5' arrow tower, so you have 3/4 cover, and are shooting at your enemy through a slit so as not to give away the insubstantial nature of your defense.
Additionally, you may want to consider Silent Image, since it is bigger, and can create a "visual effect" not just an object (like a pea soup fog), again, allowing you to avoid arguments as arrows can pass through fogs just fine.
There is no clear cut rule for this, so the DM must make a ruling. There are several ways to go about it that would make sense:
Personally, I'd probably go with #1 for simplicity. However, if you shoot or cast a spell through the object, I would allow anyone seeing it to immediately know it's an illusion, since they see something going through the illusory object, which is called out in the spell description as revealing it.
Jeremy Crawford also supports this interpretation:
Minor illusion can create an illusory object that is big enough for you to hide behind or within (assuming you're not Large+). However, if you're taller than 5 ft., you'll often have to be effectively prone if you're trying to attack from inside the illusion.
I allowed this in one specific case; two players had found a secret compartment in a dungeon, covered with plaster, which they ripped away to reveal a small entrance into a secret room.
I allowed them to create an illusion of the plaster still being present, to ambush a patrol. It was a 5" square sheet of plaster. I allowed it because there was space for both players to shoot an arrow through the illusion, in a surprise attack.
If they created a crate, the guards may have realised that something was up, as the crate was not there a few hours earlier. However, if they guards were not familiar with the dungeon, they may have tried to open the crate, only to receive the rogue's rapier somewhere uncomfortable.
I would rule that it does work, once. But I would also require the character to give thought to the type and location of the illusion. A simple black square in the middle of a hallway the enemy is familiar with might, technically, provide cover but most creatures will probably be able to determine that it is an illusion and see right through it (advantage on their investigation check?). But making an open doorway appear to be a closed door or adding a bush in the right spot in an area full of similar bushes could work, once.
If LOS is the problem, the illusion could be OVER the character (i.e, in his square), thus no creature could see him crouching behind it. While the examples provided for minor illusion (chair, muddy footprints, small chest) don't seem to be large enough to cover/conceal a medium sized creature, the 5 foot cube restriction certainly is.
Here is the problem that none of the other answers address.
You cast the illusion, now the caster has no special privileges over and above anyone else so until you physically interact with it or use an action and pass an Intelligence (Investigation) check, you can't see through it. The advantage you gain by being unseen is offset by the disadvantage you get by not being able to see your target.
Now, you may argue that you know its an illusion because you cast the spell. So what? Knowing it is an illusion is not one of the methods listed that enables you to see through it.
Now with enough time, you can physically interact with it and then put your plan in action so it is a good spell for a planned ambush but not so good for an ongoing melee.
Personally, I don't see any game balance issues with this since it effectively using a cantrip to give the same benefits of a different cantrip (True Strike). I certainly like the 5e treatment of illusions which bring back some of their power and versatility that has been nerfed out over 3rd & 4th editions.
I think there's 3 important interpretations that determines how Minor Illusion interacts with cover, hiding, and so forth.
If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.
If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.
Physical interaction with the image reveals it to be an illusion, because things can pass through it.
I think the phrasing of 3 is what matters most. And I think the correct interpretation is B. If you see someone walk through an illusion, you know it is an illusion, and the wording doesn't use "you" or "they" to specify only the actor knows it is an illusion. Also it just makes the most sense.
This implies that 2 also is B because you can discern the illusion is fake by physical interaction, or observing someone's physical interaction, which does not require Investigation. Simply seeing something moving through the illusion is sufficient.
And this implies that 1 is B as well. Investigation is just one way to contest the illusion and reveal its nature, rendering it "faint", or otherwise neutralize its effects fooling a creature.
So what does that mean?
So you would not get multiple Advantage attacks from a single Minor Illusion Cantrip, without peeking around it and avoiding physical interaction. Peeking to attack means taking the Hide action like with any other cover if you want Advantage.
Spells without any kind of visual ranged attack could be cast repeatedly without revealing the total cover to be an illusion, though without Subtle Spell or similar, it would do nothing to hide that someone was casting spells behind the illusory cube or wall or door.
There's a lot of flexible mechanics for actual utility with Minor Illusion beyond just flavor or roleplay effects.
A way you could use it would be to create a small illusion of a rock formation around you. This would work on two conditions: 1. Your character is small enough to be concealed within the illusion and 2. It is out in open fields, or in a cave.
You could also use this to create a box around yourself, or an illusion of the area around you forming a hill, inside of which you are crouching.
You could also use this to cover up a pit you and your party have dug, or hiding yourself behind a "wall".
Anyway, I am yet to try out these tactics so feel free to tell me if they would not work.
I'm pretty sure if you eliminate Meta Gaming then you as the caster can create a wall that blocks all sight of you. Do you guys have to make a hide check if you walk around a corner? What if you fail the check? If I'm on the other side of a corner can you still see me? Do you see what I'm saying? The way I use minor illusion is to conjure a brick wall and step out to fire a spell or crossbow bolt then use my movement to get behind it again. That way I'm not revealing that it isn't real. The enemy doesn't know it's not real. How many enemies have you ever DM'ed that stopped and analyzed a conjured wall? None, right? Characters aren't going to just know it's not real. They aren't going to knowingly fire a crossbow bolt into any random conjured wall to see if it's real. Maybe if the party is known for doing this then possibly. Word gets out among the bad guys that you use this. Also a shot fired may miss a target standing through it on rare occasion and pass through the wall. That would also reveal that it isn't real. But if I use Mold Earth are you going to use an action in combat when being hit by a pissed off barbarian to study the structure? Highly doubtful. My point is use common sense and stop meta gaming. If your DM does meta game you then use mold earth to create the block occasionally. If that they treat correctly, but then ignore the illusion then you have a crappy DM
