Booming Blade is a fascinating cantrip. Between somewhat murky language and unclear use-cases, there's a lot of confusion among the 5e community about how exactly you're supposed to use the Asmodeus-blasted spell.
Booming Blade's description reads as follows:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. On a hit, the target suffers the attack's normal effects, and it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.
Most of the confusion about its use cases stems from the following fact: if you're in range to use the cantrip on an enemy, then they're in range to hit you, so why would they ever move from their position, thereby triggering the damage? This leads to all kinds of theory-crafting on how to force target of the spell to move, which inevitably leads to strategies that are either too difficult or too costly (in terms of action economy or combat resources) to be practical.
So, how can a player effectively use Booming Blade in combat?
In order to narrow down potential answers and prevent opinion-based responses, the following criteria define "effective use" of Booming Blade:
- The cantrip is used in a way that guarantees one of the following:
- The target suffers the secondary portion of the damage.
- The player is able to influence the battlefield in such a way that increases their odds of success in some way that does not involve directly dealing damage via the cantrip; e.g. forcing the enemy to move into a desirable location.
- The usage of the cantrip is "economically efficient" with respects to action-economy; i.e. the player is not reducing their effectiveness by choosing this Action over another.