Movement and Reactions

[Catullus64]

I was reading old posts on another forum discussing rules interactions between certain feats and spells. It was established that a character with Polearm Master and War Caster could, if using a Quarterstaff, or another pole weapon + Spell Sniper, could use their reaction when an enemy enters their reach to cast Booming Blade.

In the scenario where you do this, how does the movement which triggers the Opportunity Attack overlap with the movement which triggers Booming Blade's secondary damage? Does the damage automatically apply, since, definitionally by the triggering of the reaction, the target is moving?

In terms of more broad applicability, there are plenty of other spells that trigger damage or some other effect when a target moves: Spike Growth, some Glyphs of Warding, spells that create difficult terrain, etc. Any caster, feat or no, can cast with their reaction in response to movement if they use the Ready action. This question could potentially come up a lot. Since movement in D&D is less of a single discrete action and more of a continuous "resource" that gets expended over the course of the turn, it's tricky to determine how it fits in with the rest of the action priority system.

Reading the PHB, it gives two points of guidance: Opportunity Attacks occur immediately before the target leaves your reach, and Readied actions occur directly after the trigger. Consider that when suggesting rulings.

[Lunali]

For the booming blade OA, the spell occurs before leaving reach normally or after entering reach for PAM. If the target continues their movement after getting hit, the additional effect goes off.

For the readied spells, the creature moves however much is necessary to trigger the condition, the spell goes off, the creature gets to decide what they want to do from there.

[Catullus64]

For the booming blade OA, the spell occurs before leaving reach normally or after entering reach for PAM. If the target continues their movement after getting hit, the additional effect goes off.


This part still kinda bothers me. A (Normal) opportunity attack happens before the creature leaves your reach; clearly, they have to still be within your reach for you to be able to target them with an attack. But by the very fact that they have provoked an opportunity attack, they leave your reach. Before the attack resolves, they are within your reach, and after it resolves they are not. Ipso facto, they've moved after the attack resolves. Maybe the general case isn't quite as thorny as I initially thought, but there's still weirdness about opportunity attacks, and therefore all spells that could be cast with a War Caster reaction.

[Lunali]

This part still kinda bothers me. A (Normal) opportunity attack happens before the creature leaves your reach; clearly, they have to still be within your reach for you to be able to target them with an attack. But by the very fact that they have provoked an opportunity attack, they leave your reach. Before the attack resolves, they are within your reach, and after it resolves they are not. Ipso facto, they've moved after the attack resolves. Maybe the general case isn't quite as thorny as I initially thought, but there's still weirdness about opportunity attacks, and therefore all spells that could be cast with a War Caster reaction.

They don't move as part of the attack, they move after the attack resolves, otherwise sentinel would end up with creatures at speed 0 out of melee reach.