Yes, I get that. So far Fitz has done a decent job of offering a fictional justification, and you haven't.
Thanks! Though I think Hriston is trying to say that
it doesn't matter why there's disadvantage until after the attack hits or misses, and then you can use whatever method you like to decide why it hit or missed. Generally you can worry about it after the fact.
OTOH, it's been pointed out here that it
would matter in the (I expect rare) case that an invisible creature is hiding from a PC, and the player asks the DM why he has disadvantage on his ranged shot. (I would still go with "trust me" myself, but trust can be a fragile thing in Player-DM-interactions).
I don't think I was assuming anything. I explained what fictional/simulative reality the rule (a hostile creature within 5' imposes disadvantage on a character making a ranged attack) appears to me to be intended to represent, and invited you to offer an alternative. The baseline rule appears to be written assuming that the nearby hostile creature is not hidden.
Maybe. But it doesn't
have to be only that. It can be
anything the invisible guy can do to screw up the shot that doesn't reveal his location (or
does, as that can also be worried about after the fact. You could
build to the invisible guy being revealed over the course of a couple of rounds:
Bob: "I shoot the zombie with my crossbow!"
DM: "Roll with disadvantage."
Bob: "Why?"
DM: "I'm sure you'll find out. Trust me."
(Bob rolls, misses)
DM: "Your quarrel inexplicably deflects to the left just after it leaves the strings."
Bob: "Wha!? What happened?"
(Here you can throw in a free Perception vs Stealth roll if you like, or you can use the "search" rule that requires it to be an action on the PC's part). Either way, in a subsequent round, either by attacking, casting a spell, being found by a player actively searching, or failing a stealth check vs passive perception, the invisible Necromancer will eventually show up. You've built some tension and made invisible Necromancer a bit scary. That's a win, IMO.
DM (later, if there's still any confusion): "It was the necromancer. Blocked your quarrel with his staff."
Bob: "Oh! Cool. Makes sense."