Maxperson
Morkus from Orkus
They cannot stop, because they have already left reach. The trigger for the AOO is, "moves out of reach," not "is about to leave reach." The attack happens right as the enemy is leaving, but that attack is not triggered until the enemy is out of reach. It interrupts the movement only to allow the attack, not to allow the enemy to change it's mind. Essentially the enemy is in mid step when it gets hit and then the step completes and it is in the new square. At that point the enemy can walk back or do whatever.Well unless you have forced movement on that AO then they can stop or move right back where they started utilizing the same movement. The rules for AOs are about the attempt not resolution of movement. Its worded like crap yes but editing is a nightmare throughout 5e. It's the shield spell paradox all over.