Mirror Realm

The Mirror Realm is a small biome that you can get to by using a Cursed Mirror. It resembles the area in which you used the mirror at. However, if there are NPCs or bosses nearby when you use the Cursed Mirror, they will not appear. Enemies will appear, however, and they will have doubled stats and a reversed color scheme.

If you happen to walk for a bit, you can see that the area is surrounded by purple fog. Stay in the Mirror Realm for too long, and the purple fog will begin to close in on you. Touching the purple fog will kill you and send you back to the normal world. However, there will be no gravestone.