Diablo/Halls of the Blind

Find the book at catacombs level 7. It will talk about some halls and creatures that lurk in the darkness. Yet in this level, or maybe even close to the book, find a room that looks like an eight and enter it.

Be careful – the creatures the book was talking about are Diablo's minions that will only appear when close to you. While far, they will stay invisible. This isn't a hard quest, simply kill those demons and, somewhere dropped in this room, get the unique item: Optic Amulet. It's not worth the price to keep it, so sell it and earn some money instead.