D&D 5E [Rules Question] Opportunity Attacks and Total Cover

[Thaumaturge]

Saturday night, my players in the Ruins of Undermountain encountered a xorn. While they were fighting, a discussion about Opportunity Attacks arose.

The xorn moved from a position where he was surrounded (PCs are A, B, C, and D; xorn is X):

A B
X C
. . . D

To a different position:

A B
.. C X
. . . D

The xorn made this movement using his burrowing speed and Earth Glide ability. Does character A get an Opportunity Attack on the xorn?

The rules on Opportunity Attacks:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction
to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature
leaves your reach.

He moved 5' down into the ground, which is, technically, in reach of A, B, and C, and then he moved out of A's reach, but then had total cover.

However, OAs happen before the provoking creature leaves the threatening character's reach, so an argument can, and was, made that A should get the OA as the xorn is going into the ground.

I made a call, but allowed that I could be swayed. What say you, EnWorlders?

Thaumaturge.

 

[Paraxis]

I would rule the PC "A" gets an attack against the Xorn but at 3/4 cover so a +5 to the Xorn's AC.

 

[Thaumaturge]

Interesting, so a swipe at him just as he goes into the ground? I kind of like that. It at least gives the player a shot. There's no way the player would have hit short of a crit in that case, but it's interesting, nonetheless.

Thaumaturge.

 

[SilverfireSage]

I would rule that they would get the opportunity attack, and I think it would be fine to give the Xorn some sort of cover, but I also think it would be fine if he didn't get it as well. My thought is that burrowing into the ground takes concentration and effort above and beyond being able to still actively defend against oncoming attacks. An opportunity attack would be warranted then, and as it interrupts the movement, would happen before it was able to get much cover, right after it dropped its guard to start burrowing, and right before it started to burrow. But in either case, it would be reasonable to give an opportunity attack.

 

[Dausuul]

I would rule the PC "A" gets an attack against the Xorn but at 3/4 cover so a +5 to the Xorn's AC.

I might use disadvantage instead, but I agree with the general idea here.

Strictly by the rules, the xorn does not provoke (or rather, it does provoke, but the attack can't be made due to the ground being in the way). However, I think it's reasonable to give the PCs a shot at whacking the xorn before it dives.

 

[Thaumaturge]

I might use disadvantage instead, but I agree with the general idea here.

Strictly by the rules, the xorn does not provoke (or rather, it does provoke, but the attack can't be made due to the ground being in the way). However, I think it's reasonable to give the PCs a shot at whacking the xorn before it dives.

I don't mind this ruling. It's a very "say yes" DM move.

Does anyone have a problem with only A getting the OA? It feels weird to me that the xorn is disappearing for all 3, but only 1 gets the OA.

Thaumaturge.

 

[Paraxis]

I don't mind this ruling. It's a very "say yes" DM move.

Does anyone have a problem with only A getting the OA? It feels weird to me that the xorn is disappearing for all 3, but only 1 gets the OA.

Thaumaturge.

I was just thinking the same thing. But if the creature say used flight to move to the same place it would be clear that only A would get the attack of opportunity so I am fine with that. Same if it was a tiny creature that moved through player C's square.

 

[Dausuul]

I don't mind this ruling. It's a very "say yes" DM move.

Does anyone have a problem with only A getting the OA? It feels weird to me that the xorn is disappearing for all 3, but only 1 gets the OA.

Thaumaturge.

I was just thinking the same thing. But if the creature say used flight to move to the same place it would be clear that only A would get the attack of opportunity so I am fine with that. Same if it was a tiny creature that moved through player C's square.

The problem with having only A get an opportunity attack is that it depends on knowing the xorn is going to stay within reach of B and C while it's underground; and you don't know that. The xorn's movement is not predetermined. Suppose A crits on the opportunity attack and knocks the xorn down to 1 hit point; the xorn could change its mind about resurfacing to attack D, and could instead just sit where it is, safe under the floor. On later rounds, it could run away without ever resurfacing at all. In that case, there is no basis for A to have an opportunity attack but B and C to be denied one.

I think whatever opportunity attacks A gets, B and C should also get (with the same penalties; -5 to hit, disadvantage, whatever). Diving underground would be considered equivalent to moving out of reach, albeit with a penalty due to its unconventional nature.

 

[Joe Liker]

The problem with having only A get an opportunity attack is that it depends on knowing the xorn is going to stay within reach of B and C while it's underground; and you don't know that. The xorn's movement is not predetermined. Suppose A crits on the opportunity attack and knocks the xorn down to 1 hit point; the xorn could change its mind about resurfacing to attack D, and could instead just sit where it is, safe under the floor. On later rounds, it could run away without ever resurfacing at all. In that case, there is no basis for A to have an opportunity attack but B and C to be denied one.

I think whatever opportunity attacks A gets, B and C should also get (with the same penalties; -5 to hit, disadvantage, whatever). Diving underground would be considered equivalent to moving out of reach, albeit with a penalty due to its unconventional nature.

I would absolutely not grant an opportunity attack.

An important part of the xorn's power is the ability to maneuver unimpeded. It's what makes the monster a threat. Otherwise you're just fighting an ogre who sometimes rolls around in the dirt. (OK, so they all do that all the time, but you take my meaning.)

I'm a big proponent of "say yes," but not when it guts the core feature of an otherwise terrifying monster.

 

[Boarstorm]

The problem with having only A get an opportunity attack is that it depends on knowing the xorn is going to stay within reach of B and C while it's underground; and you don't know that.

Perhaps not. But you don't know that he isn't either (okay, I got lazy in parsing negatives. You know what I mean, I think?).

So the fair way to do it would be to go square-by-square. The first square, A would get his OA. The second and third square, B and C do not because they are technically still in reach. The fourth square, B and C MIGHT depending on whether the Xorn surfaces (still in B and C's threat area) or continues to moves away -- in which case they can't anyway because line of effect is blocked by total cover.

I don't really feel like B or C would get an OA in any of these cases.

TLDR: You don't decide OAs at the beginning of a creatures movement, only when he would provoke one.