Seriously, We heard of them before but who are they exactly?
There was theorys about 
being a watcher but riot confirmed he wasnt
Edit: Whos side are they really on? What is their purpose in runterra? Who are they mostly interest for?
While I don't have a direct answer, I have been wondering about this myself and I've done a little bit of theory crafting if anyone's interested. Disregarding whether or not Vel'koz is a watcher, I realised that their goals could well potentially align.
Do you remember how Vel's lore excerpt went down? He wound up discovering Urtistan, and was notably fascinated by the prospect of altering the flow of time. We don't know a great deal about the frozen watchers, but what we do know is that they, and Lissandra, want to encase the world in ice, essentially stopping the flow of time completely. If Vel'koz were to attain such a power, or if he were to develop an immunity to the effect and move around on the frozen world freely, he'd be able to study the creatures and artifacts of Valoran to his wriggly purple heart's content. In fact, if he were to discover a means to circumvent the frozen world, and then share that means with the other creatures of the void, then their eventual takeover could proceed completely unhindered.
What I'm getting at here is that if Vel'koz isn't a watcher, or even some remnant or potential descendant of them (not sure how that would even work), then he could still have some sort of connection to them by trying to discover and develop their methods to his own ends. That is, if in fact the watchers don't already want the exact same thing for the exact same reasons. I'd be excited to see some form of confrontation between Vel and Lissandra in the future.
