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.
Best Answer
As a Readied Action
One of the problems that melee combatants face once they get Extra Attack is that Extra Attack is only usable on your turn. As such you cannot use Extra Attack as a readied action. What you can do, however, is ready Booming Blade and take advantage of the extra attack damage at higher levels.
To encourage the enemy to deal with you
If you are a frontline fighter (such as an eldritch knight, Hexblade bladelock, high elf barbarian) you might prefer the enemy attack you rather than go around you and attack the squishy caster in back. Depending on your build you might do more damage with an extra attack than with Booming Blade even if the target moves. What Booming Blade offers, however, is a way to incentivize the enemy to stay put. Sometimes even a regular wizard/sorcerer/warlock might want to be a substitute tank if they managed to avoid damage in earlier combats and the regular tank is low on health.
To prevent the enemy from fleeing/disengaging
There are many enemies that can retreat without provoking opportunity attacks. This can be especially inconvenient if you cannot just follow them (they can fly, they are faster than you). Booming Blade help keep them nearby so that you can attack them next round.
To harm the enemy if it follows you
This is especially applicable for Arcane Tricksters who can Booming Blade and then disengage but it can be also helpful for someone who is willing to risk an opportunity attack to keep the enemy from following. For example you are a warlock facing a bandit. You could disengage but the bandit will just follow you. You could dash but so can the bandit so you will end up in the same situation. But if you booming blade and then retreat and the bandit stays put you can then dash on the next turn and the bandit will be unable to catch up.