This quest is not available in game.
Bring the books Spells of Shadow and Incantations from the Nether to Varimathras in Undercity.
Many denizens of the Legion bestow gifts on their followers as you may well know. Some of these gifts the Dark Lady feels would be better off in her hands than in the hands of those with... lesser vision.
A sect of the Shadow Council hidden in caverns below Orgrimmar known as the Searing Blade are a prime example. The Lady tells me that members of the Searing Blade have come into the possession of two powerful spell books--she would like to "borrow" these books from them. You will get them for her.