Avenger's Breastplate

That cult would never die till the stars came right again, and the secret priests would take great Cthulhu from His tomb to revive His subjects and resume His rule of earth.

From The Call of Cthulhu.
Compare to:

The terrible fate that awaits Azeroth could come in days or years; time has no meaning for beings of his power.

When the stars are favorable, he will lash out against all living beings with terrible wrath. You must find him before then and find a way to bring his existence to an end.

Likewise,

In his house at R'lyeh dread Cthulhu waits dreaming.

Compare to the progress text, which says:

At his dwelling in Ahn'Qiraj, dread C'Thun awaits in slumber.

Because, of course, C'thun's name is a whack at Lovecraft's character, "Cthulhu" and the idea of a buried, ancient, evil god is pretty Lovecraftian.
Here's another one:

"...there is no language for such abysms of shrieking and immemorial lunacy, such eldritch contradictions of all matter, force, and cosmic order..."

When you beat C'thun, your character is told:

"...By slaying C'Thun you've not only saved all of Azeroth, you've also tipped the scales of cosmic events beyond your comprehension..."

So rejoice heroes. You're affecting the entire universe with your actions and defeating an evil based on one of the most fearsome villains of all fiction.

Gratz.

This quest are repeatable and still grant 500 reputation per turn in.