What creature counts as “an enemy creature” in terms of the Holy Nimbus

Question

Oath of Devotion has the "Holy Nimbus" feature, which are supposed to damage all enemies in range:

Whenever an enemy creature starts its turn in the bright light, the creature takes 10 radiant damage.

What is "an enemy creature"? AFAIK, there is no such term in D&D.

Spells and feature descriptions have more precise criteria normally:

• "each creature in an area" - when it's an AoE
• "unwilling creatures" - when it's an effect that can be recieved voluntarily
• "choosen creatures" - when targets are supposed to be explicitly chosen by the caster

Spells never affect just "enemy creatures". What if the Paladin doesn't think it is enemy? What if he/she is unaware of its presence?

Answer 1

"Enemy creature" is used in several instances, most of which are Paladin Capstone effects, but also for the spell Sanctuary. There are also several instances of "hostile creature" which is likely an equivalent term (but also is undefined).

While those terms aren't specifically defined, neither is "a willing creature."

In the case of these three examples, the terms are self-explanatory. An enemy/hostile creature is one who is actively against you(usually combat-related, but this other Roleplay possibilities exist), while a willing creature is one that is actively with you.

Knowledge of their presence is not required - the creature simply needs to be within range and qualify.

Answer 2

Since it's undefined, it's up to the DM to decide. A good definition of a paladin's enemy is any creature that either the paladin considers to be an enemy, that considers itself to be the paladin's enemy, or that would attack the paladin without compunction if it knew of his presence. Regarding knowledge of the creature's presence, that's not part of the aura's rules text and appears to be immaterial.

Answer 3

The magic "just knows." That's why it is magic.

Neither the caster, nor creatures need to be aware of intentions or belief, because the GM,as the personification of the universe, knows if someone is an enemy or not, by whichever rules "friend" or "enemy" is defined in your game's metaphysics.

This could provide some interesting role playing options if an enemy spy were pretending to be the caster's ally, and was fighting alongside them in combat (clearly an 'ally'/'friend' in terms of pure combat behavior) were planning to betray them later. Would that character take damage? I'd say yes, because the magic still "knows" they are an enemy regardless of their current friendly behavior.

"Your Holy Nimbus envelopes the group in sustaining, warm protective light. The demons are driven back, screeching before the blinding power of your Deity. Oddly, after the battle, you notice that Frank the Helpful Healer you picked up back at the tavern seems to have a really bad sunburn. Weird."

Answer 4

An enemy creature is a creature who is your enemy.

In D&D 5E, words mean what they mean. There's no jargon or technical terms or special meanings, just natural language. So, don't overthink this.

Any creature who opposes you will get damaged by Holy Nimbus. Any creature who is willing to be turned into a t-rex does not make a saving throw against polymorph. You can bless any creature who is on your side and curse any creature who is on the other side.

As to how the spell knows the motivation of possible targets, well, it just knows. Effectively, magic spell targeting comes with a built-in augury.

Mike Mearls references natural language in several publications and podcasts. For example, talking about the word "target" in http://dnd.wizards.com/articles/features/wolfgang-baur-girl-scouts-midgard.