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I'm in an argument with a player at my table, he says hiding behind some bushes while shooting arrows at my monsters gives him partial cover because they can only see his head and shoulders. So they're limited in what they can even aim at. I accepted that. And now he's trying to use minor illusion to create fake bushes/walls and continuing to claim he has partial cover.

I thought cover was based on having a solid obstacle between you and the target... Like, if a magical attack, or penetrating attack can pass through your cover, wouldn't that negate the AC bonus from partial cover? Or is it about "aiming"?

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    \$\begingroup\$ For a real world comparison, to justify your ruling as a DM, consider that in the Army, we were taught the difference between cover and concealment. Cover physically prevents you from being hit. Concealment prevents you from being seen. A five-inch thick wall of bullet proof glass provides excellent cover, but no concealment. Wearing dark clothes in the woods at night provides excellent concealment, but no cover. Standing behind a huge boulder provides both. Wrapping yourself in plastic wrap provides neither. \$\endgroup\$ – Kevin Jul 17 '18 at 18:46
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    \$\begingroup\$ I'd mention that as the DM you should simply stop arguing. You will hear the pleas of those you are playing with and then you will be the Judge; which is to say that it's your decision. This can be abused, sure, but I remember early on when I had someone constantly saying "you can't do that!" when I was learning to DM I basically told them "The rules defer to me in all cases" and it helped to solve it. We still discuss things and I'm amenable to "fixing" things, but I have to figure out 100's of pages of rules on the fly even when prepared, arguing RAW to me in the session is no good. \$\endgroup\$ – blurry Jul 17 '18 at 19:41
  • \$\begingroup\$ @blurry Unfortunately I'm DM'ing for a group that includes a rules lawyer and another DM, and they both think they know best, thus I have to come up with citations, or they'll never stop giving me grief. \$\endgroup\$ – AshRandom Jul 17 '18 at 22:04
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    \$\begingroup\$ @AshRandom, a good way to deal with on-the-fly calls it is to just make something up quickly and keep the game moving. Personally, I give 60 seconds for discussion. After the session you can look up the rule and play it differently in future, but that sessions on-the-fly decision stands. \$\endgroup\$ – Greenstone Walker Jul 17 '18 at 22:52
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No, but it can provide obscurement.

Cover requires a solid object. Examples from the definition:

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.

The game mechanic that represents the sight-blocking effect of something like an illusion is obscurement.

A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

Note that the main effect of the blinded condition is disadvantage on attack rolls, not a change in the target's AC.

What if the attacker knows it's an illusion?

Minor illusion, silent image, and major image all specify that a creature that discerns that it's an illusion can see through it. If it's an illusion of, say, a wooden fence, this will happen the first time an attack hits the illusion. On the other hand, if it's an illusion of something like dense fog, an arrow or other object passing through it is perfectly normal. A creature can always spend a turn examining the illusion to make an Investigation check to see that it's not natural, but it's basically impossible not to metagame that.

Even after the enemy discerns the illusion, it is still there being a distraction. I'd downgrade it to "lightly obscured", which just imposes disadvantage on Perception checks in the area, but that's a judgment call.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Rubiksmoose Jul 2 at 20:55
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No AC bonus, but can gain total cover

In the section on Cover (PHB p. 196, italics mine; the bold is in the original text):

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle.

An illusion is not a real obstacle, and would not block attacks, so the +5 AC you would have from hiding behind a stone wall does not apply because the stone wall does not actually block more shots.

However, also in the section on Cover (italics mine; the bold is in the original text):

A target with total cover can't be targeted directly by an attack or spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

The difference between three-quarters and total cover here is the word "concealed". An orc who does not see the archer because he is crouching behind a 5'x5' stone wall won't just shoot the stone wall. A creature who knows the illusion is fake, however, will cause the target to not benefit from total cover from that creature's attacks.

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    \$\begingroup\$ Given the existence of the obscurement rules, and the progression from half to 3/4 to total cover, I'd argue that the word "concealed" here is simply a mistake. No other form of cover depends on "concealment" in the visual sense, and the rule does still say "by an obstacle", which implies something that obstructs movement, not just sight. \$\endgroup\$ – Mark Wells Jul 17 '18 at 17:16
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    \$\begingroup\$ What's up with the "(sic)" marks? There's nothing wrong with three-quarters/total cover, as far as I know. \$\endgroup\$ – user17995 Sep 23 '18 at 13:09
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    \$\begingroup\$ @TuggyNE: I think the intent is to distinguish which bolded text was bolded in the PHB/rules vs. which text Blake bolded himself. I agree that it's not totally clear that that's what he means, though. I've edited it to use italics for Blake's emphasized portions instead. \$\endgroup\$ – V2Blast Sep 24 '18 at 0:58
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In the end it's your call as the DM.

Read the cover rules from the SRD.

Cover

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

Now that we have the relevant information, you may realise it's not explicit how an obstacle makes someone harder to harm. As you pointed out it could be both from not being able to see at least half of their body, or from having a physical object to possibly block the attack.

You can rule it either way.

Let's say your players are getting shot by arrows for this example:

  1. They don't get the cover bonus unless it completely conceales the character. Reasoning is even if the arrow was to hit this illusionary wall it would go through and hit like nothing happened. Hence no bonus.
  2. They get the cover bonus until one of the arrows goes through the illusionary wall (when that happens is also your call, could be the first arrow that misses, or if an arrow misses because of the +2 AC of cover could be a good one) Reasoning is that the enemies are trying to aim at the other half of their body making it harder to shoot, but once an arrow goes through the illusionary wall the illusion loses it's effect on the people who see that happen.

Talk to your players before the next session and determine how illusionary covers will work together.

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Illusions should not provide cover

Here is the first sentence about cover in the PHB, page 196

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.

Described that way, cover is really about harming the target, not aiming. The examples in the description above also mention solid obstacles, which prevent the projectile from effectively harming the target. An illusion won't prevent the projectile to get through it, and therefore it should not provide cover.

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No AC bonus, probably

The text about cover is, as usual, unclear, but it only specifies thick, solid objects as cover examples indeed suggesting that the point of cover is to provide a solid barrier stopping the attack. Examples include:

  • Half cover: low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend
  • Three quarters: a portcullis, an arrow slit, or a thick tree trunk
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An illusionary obstacle does not grant real cover, but it could still grant AC bonus against, say ranged attacks, until the attacker discovers that the obstacle is an illusion. It is also possible to consider the attacker to be at a disadvantage when appropriate.

Trying to shoot an arrow and seeing that it flies through the obstacle with no resistance will immediately cause the attacker to think that the obstacle is illusionary.

All these are based on the following tweets (1,2,3) by Christopher Perkins (one of the designers of 5e, though he is not the official rules designer, as @V2Blast reminded in a comment) on March 22, 2017 in response to questions raised by twitter users Nicolas Grinschgl and Rose Artemis:

Q: Does Illusory wall give cover bonus to AC?

A: An illusory wall provides no cover since it has no substance, but the attacker can't see anything behind it.

Q: Ah that's interesting, so if somebody hides behind a illusionary wall which only covers 50% of him, no AC bonus is granted?

A: If the attacker believes the wall is real and is trying not to hit it, the target would effectively have cover.

Consider the obstacle is a not a wall, but something that would make it harder for the attacker to see through; perhaps like the bushes in your question. In that case, it might be possible to interpret the situation the same way you handle the "Heavily Obscured" rules, which cause to the attacker to be at a disadvantage would be appropriate:

Q: This means the attacker is at disadvantage if they're trying to attack you based on sight yeah? (i.e. without blindsense)

A: You are correct!

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