You will find the True Spirit who gives you this quest in [-7,-1]

He will ask you to get The Necronomicon and 100 Arcane Essence, he will tell you that you need to obtain a key (see The Key of the Necromancer quest) to unlock the book which is found in [-5,-4] there is a secret "hidden door" between the 5th and 6th candles to get to the room...

when bringing the key in there will be loads of skellies, you would do well to get the "spirit of light pet" as it will help you to defeat the undead minions. it can be found in [-3,-6]