Lore Analysis & Theory: The Loa did speak of Sylvanas to Voljin and its for a very overlooked reason....

Let us go through History:

Around 16000 years ago the Troll Empire was born and at the height of their power they were the dominant race of the world. This [Chronicles Map](https://d1u5p3l4wpay3k.cloudfront.net/wowpedia/thumb/3/3b/Kalimdor_Chronicle.jpg/1024px-Kalimdor_Chronicle.jpg) stands testament to that. Among them were the Dark Trolls, who was a smaller tribe.

After their discovery of the Well of Eternity, the descendants of these dark trolls, now calling themselves "[Kaldorei](https://wow.gamepedia.com/Kaldorei)" in came to power. Despite the trolls' attempts to keep them from expanding their territories, the night elves built a strong empire that expanded rapidly across primordial Kalimdor. Wielding magics never before seen by the their ancestors' kin, the night elves had little trouble demolishing the trolls' twin empires and dominated the continent as [this map](https://d1u5p3l4wpay3k.cloudfront.net/wowpedia/thumb/a/a3/Night_Elf_Empire.jpg/1024px-Night_Elf_Empire.jpg) displays.

Eventually, the night elves' reckless overuse of magic lured the demonic Burning Legion to the world. The demons destroyed much of the night elves' civilization. At the end of this conflict, known as the [War of the Ancients](https://wow.gamepedia.com/War_of_the_Ancients), the Well of Eternity imploded. The resulting shockwave shattered the greater landmass of Kalimdor. The center of the continent was blasted beneath the sea, leaving a group of multiple, broken, continents. Thus, great chunks of both the Amani and Gurubashi Empires were lost to the waves.

So we can see here that the Night Elves were responsible for the collapse of the Troll Empire and brought them untold woe .... So its not unsurprising that the Patron Gods of that decimated people, the Loa would speak of someone who will bring destruction to their distant kin that robbed them of their glory. Loa possibly did speak of Sylvanas to Voljin for a reason that many would not understand ... the reason was a 10,000 year old grudge.

TLDR: Night Elves destroyed the Troll Empire, the Loa pushed for a champion that would act as the instrument of their vengeance when the opportunity arose.