| Y | - | C | - | T |

Y = You
C = Invisible Creature
T = Target

What happens if you cast a firebolt on an enemy but in the line of sight between you and your target there is an invisible creature you know/don't know about?

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    @fabian we don't use code blocks for anything except code here, not even to make formatting look nicer. See this Meta for the reasons behind it. – Rubiksmoose Nov 15 at 14:54

Invisibility doesn't affect the cover rules (and neither does (not) knowing the creature is there)

Anything that is an obstacle can provide cover for a creature. All that matters for the sake of the rules is that that thing can physically interrupt the path of whatever is trying to cross through it. A creature, invisible or not, can provide cover against spells (which require an uninterrupted line from the source to the target).

Normally, a creature counts as half cover:

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 creature, whether that creature is an enemy or a friend.

If the creature is large enough a DM might rule that they provide Three-Quarters cover or even Total cover, but the default for a creature is half cover.

Let's assume that the invisible creature gives half cover in your example. That means that if you cast fire bolt with an invisible creature between you, the target creature (not the invisible one) will get a +2 bonus to its AC, making it that much harder for your spell to hit it.

Unless the creature is providing full cover to the intended target (see next section) nothing about spell targeting or line of sight is changed.

The fact that you do or do not know about the creature has no bearing on the matter. Your DM will know and will adjust the results accordingly.

By the default rules, the creature providing cover does not get hit in any case.

Special case: full cover

If your DM rules that the creature provides full cover against the target, then you will not be able to cast your spell. To target something with a spell you must have a clear path:

To target something, you must have a clear path to it, so it can't be behind total cover.

So, in the rare case that the creature in your example is providing full cover to the intended target (meaning it is covering them entirely), your fire bolt will simply fail to cast.

If your DM is using the optional rule for invalid spell target from Xanathar's Guide to Everything, they may rule that attempting to cast this spell would still burn the spell slot.

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.

Otherwise, the rules are unclear if it would or if you simply would be unable to cast it.

Optional rule: hitting cover

There is an optional rule that your table can use that simulates the chance to hit the covering creature.

If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.

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    Doesn't xanathar have a rule that says something about not having a valid target causes the spell to cast but nothing happens or would this not apply – Jihelu Nov 15 at 16:33
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    @Jihelu good suggestion! I added a bit about that. – Rubiksmoose Nov 15 at 16:41
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    Invalid spell target or not using the hitting cover rule sure makes mass invisibility of a crowd of screening minions an interesting wrinkle for magic users in a battle setting. If both were used together it would be a no brainer for a big baddie with the spell power and the man power. – Myles Nov 15 at 22:16

According to the cover rules, the target can have cover for being behind a creature. The DM decides how much cover depending on the size of the creatures and maybe other factors. Then you can use the optional rule for hitting cover to decide if you hit cover (from the DMG on page 272):

If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.

Without the optional rule, there's no provision for hitting the covering creature, though it still provides cover.

There's a similar answer here: When creatures provide cover against attacks, can they be hit?

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    Might be worth mentioning that the "Hitting Cover" optional rule is in the DMG. – Slagmoth Nov 15 at 14:38
  • So, using this base, if i know there is a creature inside a darkness spell, a melee attack have to guess the location, but what about a ranged attack made by trying to cast for example your firebolt trought the darkness, covering not a single space but a line? Is this right or i have to guess the location for ranged attacks for a single space too? – Foffos Nov 15 at 14:44
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    @Foffos If you don't use this variant rule, it's not going to be helpful in determining your answer. If you do use it (or don't!), that'd be good to add to your question. – NautArch Nov 15 at 14:49
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    This all looks correct, but feels wrong. This theoretically makes it much easier to hit an invisible creature. – goodguy5 Nov 15 at 15:16
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    @goodguy5 You still need to beat their AC, while not beating the AC of the covered creature. – bvstuart Nov 15 at 15:54

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