Imagine you’re in a scary mansion and everything is cold and dark and covered in dust-cloths. You’re some kind of investigator or adventurer or couple driven in by a storm and a broken-down car. A monster pops out (as they do) but not from around the corner or something, but from a mirror. Cliché by now, right?
Said monster, after being shotgunned or pushed over a banister or whatever, runs over to the nearest mirror and pops into it. OK fine, you think, I’ll just go around busting all the mirrors.
So you go around ripping off dust cloths and smashing priceless heirloom mirrors with a big old candlestick. It seems like the monster is running around in its mirror-world trying to get ahead of you, because you’re so successful at smashing mirrors you stay ahead of it.
By the end of it you’re exhausted but triumphant. You even went into the basement (or sent a hireling down) to get the mirrors you know must be lurking down there. Heck, you even emptied the bathtub in case the DM says that counts as a mirror. Great work!
But now the monster is stuck in his own world. What if you wanted the monster to stay out? Like, let’s say the monster is causing problems and in order to attack it you need to lure it out of the mirror-world. You’d need to find a room with a few mirrors (so it feels safe coming out) but just enough that your party can smash them all at once. Is there a way to make a mirror you can control? Does a window between a dark and bright room count, so that you could flip a switch and remove the mirror? Would the monster be able to understand that vulnerability?
Or what if you want to enter the mirror world? Maybe the monster took something or someone in there that you want to retrieve. Could be some difficulty acquiring the correct item if the mirror-world version is still in there too. Or maybe you want to get the mirror-version of something. What is the mirror-version in relation to the regular? Is it evil and has a goatee? Left vs. right handed? Opposite magical effects? And how do you enter the mirror world – does the monster leave a slime you can coat yourself with, or do you have to grapple it and pass through at the same time? Maybe whoever owned the mansion was researching magic to pass through but it requires components / ingredients or is limited-use or requires good timing.
Was the person who made the mirrors important? Is the monster trying to drag him back through the mirror to his original dimension?