I really like the concept.
Though I don't think it should be unlocked late or be aprticularly tedious to make, there has to be a way to balance it, for reasons already mentionned before by other people.
I'm thinking of a few ways to do that, for instance :
- maybe the mirror would shatter after use, either deleting itself or downgrading itself into a Magic Mirror or Ice Mirror.
- maybe upon use in Mediumcore it will completely wipe the inventory, all of it, so if you want to retrieve your items, you either have to go there on your own with equipment from a chest, or go there naked with the Black Mirror, not both ; in Softcore it will wipe all of your coins in the inventory, it does nothing in Hardcore ; maybe instead of deleting them it just drops them ; also all buffs are removed.
- maybe there is a downside to using it, either reducing your life or a debuff reducing or preventing life regeneration for a while or maybe a debuff that weakens you or prevents you from attacking or reduces mobility ; or maybe a special debuff that lasts quite long and makes it so that if you die while having this buff, all the items that would have dropped are deleted.
- maybe the Black Mirror isn't completely reliable, not teleporting you directly at the exact last spot you died but rather within the area around it, so you you can't just go there and instantly teleport away, you have to do a very short travel to it.
The safest way would be a combination of these, the Black Mirror transforms into a Broken Mirror upon use, you need 1 Black Lens to fix it back into a Black Mirror, essentially needing 1 Black Lens per usage, which I think is fair, then you can't get any buffs since they'll be removed upon use, in Softcore, you can't pick up money from your stash before using it otherwise they'll get deleted or dropped, and in Mediumcore you can't pick up any item from chests before using it otherwise they'll get deleted or dropped, then you get teleported around the spot of your last death, generally close enough for it to be within your screen, your life will be instantly cut to 100 HP so you can't make use of the Nurse before using it, you also get Potion Sickness for 1 minute, and the Mirror Curse debuff which lasts 30 minutes which will delete all items dropped on death while you have it, the Nurse can't remove this debuff as well.
Maybe there would be minor upgrades to the Mirror, for instance using 1 Shadow Scale or Tissue Sample will make the Broken Mirror or Black Mirror into the Dark Mirror, which has a reduced chance of breaking during usage, for instance 20%, and you will have your health only reduced to 150 HP.
Maybe then upgrades for Souls of Night, another with Hallowed Bars and all three boss soul types, then another one with Ectoplasms, and lastly one with Luminite Bars. Like for 15 Souls of Night you can turn the Broken Mirror or Dark Mirror into the Night Mirror, 5% chance to break, health reduced to 200 HP, you would only have 25 minutes of Mirror Curse debuff as well.
With 3 Hallowed Bars and 1 Soul of Fright, Might and Sight and a Night Mirror, you get the Hallowed Mirror, it can't break, health reduced to 275 HP, only 20 minutes of Mirror Curse debuff.
With 10 Ectoplasms and a Hallowed Mirror, you get the Harrowing Mirror, health reduced to 350 HP, only 15 minutes of Mirror Curse debuff, only 30 seconds of Potion Sickness.
Finally with 5 Solar, Nebula, Vortex and Stardust Fragments and 20 Luminite Bars, you get the Void Mirror, it doesn't reduce your health, it doesn't give Potion Sickness, and it only has 10 minutes of Mirror Curse debuff.