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 (Illusion weavers) that will only appear when close to you. While far, they will stay invisible. With ranged attacks, you can even take out the enemies before they make themselves visible. This isn't a hard quest, simply kill those demons and, somewhere dropped in this room, get the unique item: Optic Amulet. It's up to you whether you want to sell it for some extra gold, or keep it. It increases your light radius, allowing you to see a bit more in dungeon levels.
Beware: those invisible demons have a strong melee attack.