This is about a rules bug. I did not find any other posts on this specific topic.
The above image shows the feedback from a fighter trying to attack a goblin when they are both inside a fog cloud. Attacks involving a party inside a fog cloud never have either advantage or disadvantage unless one of the parties can see through the fog cloud in some way. Targets inside a fog cloud are heavily obscured -- imposing disadvantage -- but are also blinded, which grants advantage. Any amount of advantage cancels out any amount of disadvantage. The same applies to attackers inside a fog cloud: their targets are heavily obscured to them because of the fog in the way, but neither can they see the attack coming.*
*: The wording of the vision errata is sometimes taken to mean that a creature inside a fog cloud can see creatures outside it just fine. While I think that interpretation is nonsense, I'm deliberately ignoring that scenario for purposes of this bug report.
Targets inside a fog cloud are heavily obscured -- imposing disadvantage -- but are also blinded, which grants advantage. Any amount of advantage cancels out any amount of disadvantage.
While this is true according to RAW I'm fine with having disadvantage to hit in this situation. Realistically, it should be harder to hit someone if both combatants are inside fog than if they were standing on a clear field. They still have armor and it's hard to see them.
I'd argue the same for two invisible creatures. According to RAW attacks are made at normal. But c'mon, really? (If 5e was a bit more complex, I could see these attacks being made at disadvantage but the defender loses their Dex bonus to AC to partially compensate)
Lots of D&D doesn't make sense, but is there for balance. Imposing disadvantage on both parties seems even-handed but it prevents rogues from sneak attacking.
Lots of D&D doesn't make sense, but is there for balance. Imposing disadvantage on both parties seems even-handed but it prevents rogues from sneak attacking.
I don't see why this is a bad thing or necessarily unbalanced. There are loads of things that can prevent rogues from sneak attacking; a 1st level concentration spell doesn't seem OP compared to those other things. Especially since that caster then can't concentrate on any more powerful spell.