Opportunity Attacks and Cover

Quote from VerilyRaze >>

In most games, D&D is a discrete game based around a minimum length of 5'. So if you're going to attack someone, you're doing so at 5' or 10' - there is no in-between.

If you're making an Opportunity Attack, that attack is limited by your Reach and must occur while the enemy is in a square within that reach. If you've got a 5' reach, that makes it impossible for an enemy to 'duck behind cover' before you get your Opportunity Attack. It is possible (as noted earlier) to do with a 10' Reach, but requires a specific layout.

Note that D&D does have an option for playing on a Warhammer-style board where distances are measured on a continuous terrain. This makes the question more interesting and the normal interpretation would be that the amount of cover is determined at the point where the model passes out of range.

 agree; you said it better than I could

Quote from MPA >>

OA triggers the moment they move out of your reach. It doesn't matter how close they are to total cover, a doorway, flying. If they aren't using some kind of teleport or were moved by a 3rd source, then the attack roll happens. I don't see anything that says burrowing isn't a movement type. 

Burrowing is a movement type, but if they don't have a feature that leaves a hole behind them, by the time they've left your reach, they're also underground and have total cover. Unless you assume that your reach stops when it hits total cover.

Quote from Pantagruel666 >>

Quote from MPA >>

OA triggers the moment they move out of your reach. It doesn't matter how close they are to total cover, a doorway, flying. If they aren't using some kind of teleport or were moved by a 3rd source, then the attack roll happens. I don't see anything that says burrowing isn't a movement type. 

Burrowing is a movement type, but if they don't have a feature that leaves a hole behind them, by the time they've left your reach, they're also underground and have total cover. Unless you assume that your reach stops when it hits total cover.

Is there a rule which says burrowing does not trigger opportunity attacks? Because lacking any such language, the AoO still triggers. The creature is leaving your reach, by going to the cube directly below them, and AoO's trigger before the creature leaves your reach..

Quote from Stabbey_TC >>

Is there a rule which says burrowing does not trigger opportunity attacks? Because lacking any such language, the AoO still triggers. The creature is leaving your reach, by going to the cube directly below them, and AoO's trigger before the creature leaves your reach..

 The cube directly below them is not outside of your reach and thus does not trigger the AoO. They actually pass outside of your reach when they move past 5' underground, but at that point they have total cover.

In the case of an incorporeal creature, I can see an argument that their move into a solid object is more akin to a teleport or misty step. For a burrowing creature, the creature has to turn and physically dig its way into the matrix material, so it clearly should trigger an OA.

If a DM were to rule that a Specter could phase into an adjacent wall without triggering an OA, I wouldn't feel it was unreasonable, so long as the same applied if I used Meld into Stone.  It would make for an interesting tactical engagement if there were an enamy that could phase out of a wall, attack, and then phase back in, if it was an otherwise balanced encounter. Lots of trying to avoid walls and prepping reaction attacks.