There are worlds we have not yet visited. Realms we have not seen, do not know. Some are demon-haunted nightmares, still others lost to our understanding, destroyed. The Burning Legion has been thwarted twice on Azeroth, but make no mistake - they have yet to bring their full power to bear, seeking instead to seize the world's unique magical energies for their own use. The dead world Xerrath demonstrates that the Legion has in the past destroyed entire worlds merely as a show of force to cow others into submission and slavery. What's worse is, there are races that were wiped out by them, but others who proved their worth were instead made into servants and slaves to the Legion - and Azeroth has balked their advance twice, putting us squarely in the strong enough to make good slaves category.
Yet, too, there are worlds beyond even the Legion's control, worlds dominated by the vast hosts that the Old Gods and their servants such as the Prophet Skerram, worlds like K'aresh (home of the Ethereals), worlds shaped by the Titans, and worlds that yet sleep in their infancy, unaware of the struggle between ancient inimical forces. Azeroth was one such world not too long ago. The destruction of K'aresh by Dimensius the All-Devouring proves that there are forces, hungry and ravening, that serve no masters and have destruction on their mind completely unrelated to the Legion or the Old Gods.
We know barely a fragment of the vast cosmos. Let us explore what little we do know.