Extra room at the end of diablo 1

CaLKiD

Diabloii.Net Member
I've gone back to this game after 25 years and I remember after killing diablo and getting to level 28 or 30 maybe and then being able to go to an extra room where there were doors all around and monsters would pile out of them and all the item drops were awesome. Can't seem to find it anywhere and can't remember how. Does anyone know what that was and how to get to it?
 
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