Picture the following scenario: I have an enemy in melee range who decides to move away without Disengaging, triggering an Opportunity Attack. However, I have the Warcaster feat:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.
For the spell, I choose booming blade, which has this clause:
On a hit, the target (...) becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends.
The creature, not wanting to take the extra damage, would like to immediately stop moving. I am unsure if this is actually permitted. Which of the following three interpretations is correct:
- The creature is already moving, and thus takes the damage.
- The creature may stop after moving 5 feet, taking no damage.
- The creature may stop after moving zero feet, similarly to what would happen if I had the Sentinel feat.
This question had previously been marked as a duplicate of another which did not, however, contain an answer to the posed question.