This is a fun one...DND 5E session.
Assume a 5' corridor that if 25' long leading to a dead-end (5 squares), and 5 party members (all medium size) fill each of those squares. A well armored tank as at the front, trying to defend the party. This is the scenario a party has encountered a bunch of times fighting various creatures. But never against incorporeal creatures.
The party is trying to hold off 7 specters being controlled by a Wraith.
Since the Specters are incorporeal and can pass through one another, as well as the players, as difficult terrain, are they limited to attacking the front-line only? I would rule yes, in theory, because you can't attack from an occupied space:
That rule is a fundamental aspect of the tunnel defense / choke-point strategy, and in theory even works for these non-corporeal creatures, preventing them from being able to infiltrate the middle of the party, as there is no unoccupied space for them to attack from. The PC's literally fill every available 5' square. Pretty simple and clear cut I think.
But then, there's the fact that the Specters can pass through walls...and that's where they fun begins. They have 50' of movement, so even through walls they can move 25', attack blindly through the wall, and then return 25' back. Muahhahaha. They could easily reach PC #2 and PC #3, although the two way in the far back would be safe (unless the Specter was willing to end it's round in the wall and take the 1d10 Force damage).
Seem legit? I ruled that they would NOT take the 1d10 Force Damage for stopping to attack from inside the wall, because they did not "end their turn" on that space. That's straight from the rules on Incorporeal Movement in their stat block. Do others agree? A few players weren't happy lol.
However, things quickly got so crazy complicated from here, I quickly whiched that I had ruled they simply couldn't do it. ;-)
If allowed to attack, would they attack at Disadvantage since the specter can't "see" through the wall? I ruled yes. However, since the PC can't see the attacker (it's in a wall!), the attack should be at Advantage, thus cancelling each other out!
But what if the PC saw the hand coming? You could offer a Perception vs. Stealth to see if the PC see's the "hand reaching out", and if they did, then Disadvantage would be re-asserted, right? And if so, what if that PC is also Dodging and sees the hand come out? Should the attack then be at super-disadvantage (which we don't have as a house rule and I avoid like the plague, although this one tempted me big-time).
The way I ruled it is those who were Dodging gained Advantage on their Perception check to see the hand coming. If successful, then the attack happened at Disadvantage. Therefore, there was a benefit to foregoing an attack to Dodge. However, even those who used their Action to attack still had a chance to see the hand coming if they made their Perception check, and if they did, the Specter attacked at Disadvantage. If they didn't see it coming, the Specter attacked normally as it's Advantage (attacking a creature who can't see you) and Disadvantage (blindly attacking a creature you can't see) cancelled each other out.
I'm really curious to see how others would have handled this. The party was quite shocked when their tried and true tunnel defense imploded upon them.
Also, this didn't come up, but what if a PC in the back cast Daylight? I would have ruled that the Specters in the walls would not have suffered Disadvantage on attack from it, because they couldn't see the light (it wouldn't penetrate the walls). Only the specters attacking from up front would have been at disadvantage. This would have in turn driven them all to enter and start attacking through the walls (INT 10 after all)! Major backfire potential there!