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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?

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    \$\begingroup\$ No need to signal your edits here. We have revision histories and so we just make every version of the Question/Answer the best version. Best thing to do is blend the edit seamlessly into the previous version. I have edited it out for you :) \$\endgroup\$ – Rubiksmoose Jul 3 '18 at 13:19
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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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  • \$\begingroup\$ Nice call on Identify! \$\endgroup\$ – NautArch Jul 3 '18 at 13:15
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If they have experienced it, or if they are familiar with it

Booming Blade has a visible effect (SCAG)

it becomes sheathed in booming energy until the start of your next turn.

If the target is knowledgeable of this spell or has experienced it before, then they should know that moving will make them go boom.

Determining if they know

This is going to get into DM purview here. A DM may also consider the intelligence of the creature and whether or not either of the options below are even available to determine if they are familiar enough with the spell/spellcasting to identify it:

  • They have experienced it before
  • An Arcana check

Whether or not the bottom option is viable will depend on your DM.

A DM can also reference Xanathar's for guidance in determining the DC under the Chapter 2: Spellcasting:

If the character perceived the casting, the spell’s effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell’s level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell...

This Intelligence (Arcana) check represents the fact that identifying a spell requires a quick mind and familiarity with the theory and practice of casting. This is true even for a character whose spellcasting ability is Wisdom or Charisma. Being able to cast spells doesn’t by itself make you adept at deducing exactly what others are doing when they cast their spells.

As a side note, I personally don't require reactions/actions to be used for an Arcana check and just let the PC roll for it if they ask.

What is Booming Energy anyway?

Whether or not Booming Energy is an perceivable effect also falls under DM purview. However, given that the spell is explicit in the target being sheathed in it, it does seem likely that the target perceives it (either in feeling, sound, or sight.) You could do a Perception (Wisdom) and set a DC you think is reasonable - but I'd likely rule that the effect is obvious to the target given the language of sheathed in booming energy.

We can also look to Jeremy Crawford and his Twitter response on Booming Energy:

Booming blade does make noise. The spell's description says the target is surrounded by booming energy. The DM decides who can hear the booming.

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    \$\begingroup\$ @Rubiksmoose Isn't that the feeling you get when a car goes by with excessive bass? \$\endgroup\$ – Slagmoth Jul 3 '18 at 13:05
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    \$\begingroup\$ @Rubiksmoose SONIC BOOM! \$\endgroup\$ – NautArch Jul 3 '18 at 13:06
  • \$\begingroup\$ @Rubiksmoose -- At our table, we describe it as a thin envelope of vibrating, pressurized air. If you think adding that description will improve the question, I'm happy to do so. \$\endgroup\$ – screamline Jul 3 '18 at 13:09
  • \$\begingroup\$ @screamline nah, not necessary. \$\endgroup\$ – Rubiksmoose Jul 3 '18 at 13:17
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    \$\begingroup\$ @NautArch, FYI, your answer is solid, and if I could select two, I would give you the nod as well. But the point about identify in the selected answer just seals the deal. \$\endgroup\$ – screamline Jul 7 '18 at 0:33
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As the others have pointed out: Yes, if it has had experience with the spell.

Otherwise I, as a DM, would rule anyone smart enough (even animals etc.) to know that moving through that weird energy can't be good. Ie. they don't know for sure they will get damage, but they won't be wanting to find out. If a PC assumes he would get damage I would accept that happily (maybe with a Wisdom check if he's known to be on the slow side when it comes to thinking).

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