Sort of got me thinking...
Something...?
A Facebook event page to have people storm the Void to 'see them voidlings.'
The writers.
As of now, all they should be afraid of is Lissandra and her plotting..
Existence itself - the Watchers just want to go back to sleep and reality refuses to turn the lights off.
The Watchers are already afraid. Of reality, of Runeterra. Because they lack the capacity to understand it, so like us they respond to fear the only way they can. With violence like cornered animals.
In all truthfulness, 
or a fully-free 
would probably terrify them, since they're probably part of the building blocks of the universe's existence. And if they actually teamed up together, you'd best believe the Watchers'll be scared.
EDIT: Judging by the votes it looks like being in all truthfulness isn't why we're here.
Kai'sa, for the thot is almighty and her cleavage is powerful enough to scare even the mightiest of beasts. Fear Kai'sa.
yes. their greatest threat and arch-nemesis.
The long forgotten "avoiders"
(partially due to the fact that no one has ever seen them and they are thought to just be a made up myth)
You Can see in the presentation page of the new lol book that runeterra is between the void and the celestial realm. These 2 powerful being in these2 different dimensions want to control the material realm and watchers don t attack directly aspects and aspects don t attack directly watchers. I think they fear the other and they are equal as perfect opposites But it s Just my ideas
The spells hiding in Ryzes scroll is a good one.
World Runes most likely
Teemo with prep time
Probably GrossGore, he would ask them "NAME A BATTER TF THAN ME BRUV" until they calmly peace-out
Zoe pet dragon.
He is rude and arrogant but he is making the Voids job harder
Salt is kinda scary as well. It was used to dry the lands around Shurima so the void wouldn't spread from Icathia.
abyss watchers> watchers
I trully believe that The Watchers should be afraid of the Voidborn, because eventually the Voidborn will realize that the goal of the Watchers will end up destroying them as well.
The Aspect of Targon are fighting against the void. Their pet Aurelion Sol can close void gates.
Ryze in his latest cinematic destroys a watcher by himself. Which is... pretty badass.
Humans themselves. Two humans (Kassadin and Kai'Sa) survived being in the void and gained power FROM it.
Darkin don't give a damn about anything now. I guess Aatrox would be kinda happy to have an endless fight if someone would have the idea to throw him in the void. Same for Rhaast. As for Varus I don't know. The darkin part of Varus is not very expressive.
:"Me for I am a MAN!!!!"
(cuz shes in the do not touch on the sexy tierlist and the void dont know how to not touch #TheyGoinToJail) 
JUSTICE
OR
because well their all pretty crazy
An allied
