Supposedly they are the rulers of the void, implied to be the most powerful void creatures.
The really strange part is however that they at some point appeared in the Freljord, gifted humans there the iceborn "ascension" and made them powerful in exchange for their servitude.
They also taught the humans how to build and create various things for them.
But then the iceborn under the leadership of Avarossa and Serylda rebelled and somehow they actually won a extended war and drove the watchers into the howling abyss.
Worth noting that the iceborn leaders survived and enough others also survived for them to have descendants scattered around and about over Freljord.
Meanwhile shurima´s sunborn ascended kept having entire armies wiped out by the inferior voidlings and their commanders.
Feels like the powerbalance here is really weird, both the sunborn and iceborn are ascended humans and yet the Iceborn casually kicked the voidlords and all their guards into a "bottomless" abyss that they could not get out from.
Then the Iceborn went and had children and civil war, like it was just another day.
Then we have Shurima in which multiple sunborn had PTSD over voidborn battles, massive deathtolls and areas that were devestated in battle.
Somehow its all really weird, why were the iceborn able to win so "easily" when the sunborn had so many problems?