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 SentinelDragonmark, which allows you to shift 1 square before or after an opportunity attack, or even
Deft Footworkthat 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?