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Rules Compendium p.195 states:

If an interrupt invalidates a triggering action, the triggering action is lost.

It also states on p.197 that Opportunity Actions Interrupt the triggering action.

What happens if the triggering action is a movement and the Opportunity Action taken invalidates the movement by blocking the destination square that triggered the OA?

For example: the ranger power Fading Strike can be used in place of a basic melee attack when making an opportunity attack, and this power allows the ranger to shift 2 after the attack. A monster moves away from this ranger, triggering an OA, and the ranger responds with Fading Strike, then shifts into the square the monster was going to move into.

EDIT: NEW EXAMPLE I made a mistake with the Fading Strike power example because it doesn't let you end the shift adjacent to an enemy. Consider instead the Mark of the Sentinel Dragonmark, which allows you to shift 1 square before or after an opportunity attack, or even Deft Footwork that lets you shift 1 square instead of making an attack. The point of the question was to discuss interruption of movement more than the particulars of that ranger power.

The action that triggered the OA (monster moving into a square) is invalidated as a result of the OA (an enemy now occupies that square). This seems to satisfy the conditions to completely end the move action the monster took.

Does the monster lose its entire movement action? Or can it move in a new direction? If it can move in a new direction, does it move its full movement, or has it lost 1 square in the attempt to move into the square it can now not move into?

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My D&D's a bit shaky. You certainly can't walk through an enemy-occupied square, but can you shift through an enemy-occupied square? That said, you could shift diagonally around him twice. Blocking the enemy's movement would definitely end the move as the enemy can't walk through your new square, but you must predict exactly which square the enemy is fleeing to. –  Hand-E-Food Jul 13 '12 at 5:57
    
@Hand-E-Food you can move through an ally's space as long as you don't end the movement there (if you're double moving you can end the first move there, as long as you continue). You cannot move through an enemy's square unless a power explicitly allows you to, or you're phasing. –  wax eagle Jul 13 '12 at 14:56
    
I think my last paragraph handles this issue. yes they lose the square (you've invalidated the triggering move), but they do get to continue the move action –  wax eagle Jul 13 '12 at 19:09
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3 Answers

up vote 11 down vote accepted

Ok, Let's examine the situation that would allow for this:

A monster is moving past you, allowing for an opportunity attack, towards a square that is 2 square away from your current location. As he moves past you make your opportunity attack using fading strike and move into his intended destination. This is a legal move according to both the power and the rules of movement, as he is not adjacent to the square you move into and he is not currently occupying that square. You cannot move into the square that triggered the OA, as that square is adjacent to the monster (which violates the rules of Fading Strike). However, you can move to his intended destination, but that doesn't really matter.

Here is the key, squares of movement are treated discreetly. They are a single move action, but you don't choose what your next square of movement is until you have completed the move into the previous square. After the opportunity attack the monster can choose to continue moving, and end in a different square adjacent to your character, or move in another direction entirely.

There is one exception to this. If the monster is charging he must continue his move getting closer to his intended target with each square, if you put yourself in his path along the only possible route, you stop his movement cold and negate the attack, because it is no longer a legal charge.

If you do have a power that allows you to negate the movement (such as one that allows for shifting with no qualifiers and can be used on an OA, or a push that prevents them from moving in the intended square). The square of movement that they used is gone (again, except in the case of charging, where the charge is now negated, unless they have another valid path within their remaining squares of movement)

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While I agree that squares of movement are treated discreetly, that you move square by square and can declare each square as you go, I don't think this addresses the issue of there being one whole "move action" and that the interrupt invalidates the "action". Like, if the action were a ranger's twin strike, and the interrupt invalidated the first of two attacks, the second attack would also be invalidated because the whole action was invalidated. Why doesn't this hold for movement too? –  Soulrift Jul 16 '12 at 4:30
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I believe the answer hinges on the definition of "invalidates the action", contrasting it with "disturbs the action". In my view, "invalidates the action" means the action becomes completely impossible, there's no way to salvage it with minor changes. "Disturbs the action" means the action can't be completed as described, but the same action can still be performed with minor revisions.

For instance, say an adjacent melee target moves away before the attack can be made. If the attacker doesn't have reach, and there's no other target in range, there's no way that the attack could be made, so the action is invalidated. If you do have reach, the action is still possible, just the target is in a different square. If you do have another target in range, the attack could be redirected to that target. The originally declared action, "attack with X power", is disturbed but not invalidated.

There are examples in the rules of a move action being interrupted in the middle (for instance, by spotting a trap with passive perception, or the trap's attack vs. reflex missing) but that doesn't invalidate the move, it just disturbs it and allows the player to declare different details for the rest of the action. Another example would be someone acquiring the Immobilized or Prone condition in the middle of a move or charge, no further movement would be possible and the action would be lost.

So, to answer the original question, the OA interrupts the action and disturbs it, it can't be completed as originally intended, but it is not invalidated -- it is still possible to move, just with different details. The monster gets to choose other squares to move through after the action is disturbed. I don't think the monster loses the square of movement, since it didn't take the square of movement in the first place.

This is discussed in: Can a character in the middle of a move action alter his movement based on a change in his surroundings?

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The situation you describe is not possible. Assuming the monster is taking a move action other than shifting, it triggers the opportunity attack as it tries to move into the first square away from the ranger, but before the creature moves. The ranger than gets to shift 2, but according to Fading Strike, can't be adjacent to the target, so he can't possibly be in the square the monster was moving to.

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This answer addresses his specific example, but not the rest of the questions in the post. –  Colin D Jul 13 '12 at 14:50
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