Consider a fighter (green), with a reach weapon (red), confronting an ogre (orange). On the ogre's turn, it Strides five feet southeast and then five feet east, ending adjacent to the fighter. Does this movement trigger Reactive Strike?
REACTIVE STRIKE [reaction]
Trigger A creature within your reach uses a manipulate action
or a move action, makes a ranged attack, or leaves a square
during a move action it’s using.
You lash out at a foe that leaves an opening. Make a melee Strike
against the triggering creature. If your attack is a critical hit and
the trigger was a manipulate action, you disrupt that action.
This Strike doesn’t count toward your multiple attack penalty,
and your multiple attack penalty doesn’t apply to this Strike.
[emphasis added]
Best Answer
Yes, it leaves a square and triggers Reactive Strike.
It isn't in reach when it uses the move action, nor when it moves southeast, so that part of the action does not trigger Reactive Strike. However, when it moves east, it leaves the two squares to the west of its final position. Because the trigger mentions a "creature" within the fighter's reach and makes no reference to "squares" within their reach, the fact that the squares it's leaving are not themselves in the fighter's reach doesn't signify.