Speak with Alchemist Arbington at Chillwind Camp, Western Plaguelands.
On the island of Caer Darrow lies Scholomance - a wretched center of Scourge necromancy and evil. Our enemy dwells within, conducting unspeakable acts of depravity on unwitting victims... at least, we surmise as much. A door bars the way, and try as we might no entry is to be had without a proper key.
Alchemist Arbington is well versed on Scholomance, and has an idea how to obtain a key. You have proved your worth to me, and now I entrust this task to you. Speak with him on this matter.
Well met, <class>. I've dedicated a fair portion of my recent studies to the Scholomance, and I believe I know a means by which you can acquire a key... one that will let you past the front door.
I can only imagine what horrid sights are to be seen inside, <name>. I don't know how you heroic types do it sometimes...