What and where is the mirror???

Can someone explain to me what a mirror is?
I assume its fighting yourself but please explain a bit more about it cuz ive read a lot of topics about it already...
Also where can i find this mirror?

The mirror is your final test before ascension. You fight this creature with all the same skills and attributes as you but better, i found it kinda easy since im e/me (backfire, shatter enchantment, etc) but i have heard that it is quite hard. You can find it at augry rock