A Shadow on the Horizon
http://nexus.leagueoflegends.com/wp-content/uploads/2017/06/LOL_CMS_98_Article_06.jpg
We have a bunch of new champions in various stages of development. I’m not going to go over all of the champions in production (spoilers), just the one that has been infesting my mind lately — a new carry champion that we think will perform well in the jungle. After Ivern, we wanted to try our hand at a more aggressive, damage-focused champion that should feel at home in the jungle. They will also be our first transformation champion since Gnar, but this take on transformation will be unlike any champion we have ever made. I don’t want to say much more than that for now, but keep an eye out for more info soon.
New champion is a ''they''. Whch means we will have a duo characterized champion like Kindred again.
as well 
, acquired from killing the Void-warped Rift Herald. Riot has done fairly well with diction when they tease new content, especially Champions, and the specific word "infesting" pops out. The Void has shown an ability to "infest" others, whether that be the aforementioned Rift Herald, 
's mind, and 
's story where he described the land around a Void portal becoming distorted and malformed.
On top of that, an archetype that hasn't yet made its way into this game that a fair amount of people have been wanting is a "Parasite Champion". Since parasites are mostly associated with insects and the like, the Void's aesthetics and effects make it the most suitable to lending itself to a parasitic character. The teaser also said this new Champ will introduce a brand new transformation mechanic - maybe it's not simply shapeshifting like 
or 
, but taking over a host body (perhaps a jungle monster or even another Champion). Or perhaps it becomes an amalgamation of different bodies, where it collects biomass and turns into something based on the biomass collected.
Then there's the title "A Shadow on the Horizon". There are several things in Runeterra that can be associated with darkness and shadows, the Void included. But with "horizon", that implies something not only in the distance, but something that'll bring a new dawn/dusk. If considering dawn/dusk in a more metaphorical sense, particularly a new age, then the Void can fit that bill. Every faction wants to change the world, sure, but the Void is the most drastic, as from what we can guess it wants to cause the extinction of this dimension by consuming it, turning it into an extension of the Void (as well as the Void itself being the most "distant" of anything we have, coming from an entirely different plane and whatnot). Thus, a new age for Runeterra, a "Void Age" if you will. With the word horizon I'm also just reminded of "event horizon", which is the edge of a blackhole and with blackholes theorized to be gateways to other dimensions/universes. A little far-fetched, admittedly, but as said, Riot's choice of words when doing teasers has been pretty clever, so why not try to dissect what they're implying.
or other things like that) Or it could transform by killing minions, jungle camps, drakes, nashor etc, red-buff, blue-buff and so on.. (May be a little weird tho :D)
