[RPG] Does the target of Booming Blade know that it will take damage if it moves


The description of the booming blade spell (SCAG p. 142) says only that "[i]f the target willingly moves" while the spell is in effect, it takes damage. The in-narrative effect is "booming energy" enveloping the target. Is the target automatically aware of exactly what that energy does, and the specific danger it poses?

The Player's Handbook does say, at p. 204 (and here in the basic rules):

Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless a spell says otherwise.

But does a sheath of "booming energy" constitute an "obvious" effect?

Best Answer

The target perceives the "booming energy" but doesn't automatically know the spell's effects.

There is no general rule that spell targets know when they are under the effect of a spell, and the text of booming blade gives no specific rule as such.

All the target can really know is that they are surrounded by "booming energy." How they choose to react to that is probably a matter of their intelligence and experience as adjudicated by the DM. An experienced caster or warrior might know to stay put and wait for the spell effect to pass, whereas a beast might flee in terror for lack of comprehension.

It's worth noting that the identify spell is the canonical way for a caster to ascertain what spell a target may be affected by. Since only certain casters have access to the spell and it generally costs a spell slot to cast, it's unlikely that a target is intended to know the exact details of a booming blade spell cast on them without having access to identify or a comparable feature. Xanathar's Guide to Everything includes optional rules for identifying cast spells using Intelligence (Arcana) checks, so this further implies that knowing the effects when targeted by a spell is not automatic. It's something to be adjudicated based on the context.

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