Magic exists which can divine the intentions of other creatures toward me. For instance, the description of the wand of enemy detection magic item says, in part:
While holding it, you can use an action and expend 1 charge to speak its command word. For the next minute, you know the direction of the nearest creature hostile to you within 60 feet, but not its distance from you. The wand can sense the presence of hostile creatures that are ethereal, invisible, disguised, or hidden, as well as those in plain sight.
The Arcane Archer fighter can choose the Arcane Shot option Shadow Arrow (XGtE, p. 30; emphasis mine):
You weave illusion magic into your arrow, causing it to occlude your foe’s vision with shadows. The creature hit by the arrow takes an extra 2d6 psychic damage, and it must succeed on a Wisdom saving throw or be unable to see anything farther than 5 feet away until the start of your next turn.
Abilities and effects do what they say they do. There is no fluff in descriptions.
As written, it appears that if I hit a creature with my Shadow Arrow, it does regular arrow damage plus 2d6 psychic damage. In addition, if the creature hit is my foe, it must make a Wisdom save or have its vision occluded.
Suppose I want to determine if an NPC of uncertain loyalty is my foe or not. I hit him with a Shadow Arrow. It seems that if his vision becomes occluded, he must be my foe; if not, he is either not my foe or he has succeeded on the saving throw. Is that right?
- Does the Shadow Arrow really know who is my foe, similar to a wand of enemy detection? Or does it assume that anyone I would shoot at must be my foe?
- If the latter, could an NPC that was trying to gain my trust volunteer to receive my arrow? Would that affect the result if they were being truthful vs. deceptive?
- If my DM house-rules that critical failures on attack rolls result in attacking allies when I fire into a melee, can I be confident that my Shadow Arrow will not be able to occlude the vision of anyone friendly to me?