My players went up against a homebrew creature I made, it was a fallen angel that could cast Mirror Image. Our paladin had the idea to use his Divine Sense ability to be able to tell which Image was the real one.
Before we settled how it would go about, we went into an argument that since the Mirror Image is in the same space, Divine Sense (and his Action) is wasted as the location would be the same. He countered that knowing the location of an enemy he could already see, he could point towards the real one.
I wanted to reward his clever use of the feature so I ruled that he would have to keep using his Action to use Divine Sense so that his allies could attack with impunity.
Did I call it right by Rules-as-Written?
Best Answer
Did you call it right? Sure! It's your job to bend the rules or those you don't agree with to make it fun and enjoyable and reward clever tactics.
Did you do precisely what the written rules dictate? No. Here is why:
Divine Sense
'Location' or: A creature's 'Space' PHB pg. 191
Mirror Image
Now that I've cut these down to the parts we should be paying attention to, we are left with the following conclusion with no real argument to the contrary:
A humanoid celestial angel is perhaps 2 foot across in width, but it controls a space of about 5 feet. That's its personal space, its combat space. A humanoid is not 5ft wide folks. This is its location. That particular 5ft space within its control.
Mirror Image places 3 illusory images of that same celestial in that same space. The same location. They move with the celestial, nearly imposing themselves with the celestial as to make themselves seem like an after image. Think of this as someone moving their hand in front of a high speed lens and leaving behind an after image of their hand that follows split seconds behind their real hand. A mirror image as the namesake of the spell suggests, making it impossible to track which image is real. What the spell is saying it DOESN'T do is create 3 copies of the celestial that all stand around/beside/next to said celestial like "Hi, how are you doing, which one of us do you want to hit?"
Divine Sense allows the Paladin to open up his senses and divine the location, the effective space, of a celestial that is not behind total cover. They know for all intents and purposes that there is a Celestial in that effective space, that location, but they can't identify the celestial specifically. Just that it exists.
Let's put this into the setting. There is a Celestial that seems to be moving so fast that there are 3 other images of itself that mimic its every move and action. I can't tell which one is the right one because they are so close together in its space, its location. The Paladin uses Divine Sense. "Yep, there's definitely a Celestial right there, but because the mirror images are mimicking its every move so closely together in its space (its 5ft location), I can't really help pinpoint which one is real. Swing and hope you hit something!"
Lesson: Divine Sense is helpful for finding invisible creatures of the effected creature type, or figuring out if there is one in disguise parading around in some other form. Something that counters magic, such as Dispel Magic, is better for something like Mirror Image, as are AoE attacks.