Is it the bloodthirsty Noxians? The reunion of the Darkin? A full-out Void assault? Lissandra's planned revival of the Watchers? Something else entirely?
I'm genuinely curious as to what you guys think. Could you give me your opinions?
Well I mean the Void and the Watchers seemingly. Consider the scope:
The Void consumes everything, people, animals, land, everything. Targon, a space faring empire, has the Void as a primary enemy. As the Ascended that was killed by Axamuk said, "It'll be like none of us ever existed."
Runeterra could persist with any other danger it has, other than the Void. If the Void wins, if the Watchers win, there would seemingly be no coming back.
Void/Watchers for sure. Their goal is to consume Runeterra and their power levels seem somewhat unfathomable.
Humans, humans are always the biggest danger to themselves and everyone else arround them.
The Noxians? most of them are humans Darkins? they must be wielded by humans Void? Malzahar is making it worse... and he is human. Lissandra? She was human as well... Without her the Watchers are kinda irrelevant right now. Undead? yeah, they are humans, dead humans, but humans anyways... The Ruined King was human and Morde is human. Demons? they feed of human emotions.
If you lived in Runeterra, you would be most likely to die due a human, instead of a monster... Unless you count humans as monsters.
Danger needs to be considered relative to its scale and likelihood.
-Noxus is a certain, small danger to everyone on its borders. However, Demacia is holding it back from conquering the world.
-The Darkin are a talking scythe, trapped in one body with two other guys, and a really edgy dude with a sword. Not too worried about them.
-The Void is definitely menacing everyone. Sitting there. Really menacing. They sure haven’t done much since Icathia.
-The Shadow Isles might talk a big game, but I have yet to see them talk about the giant flotilla that would have to bring them anywhere.
-Lissandra is slowly but surely trying to end the world. It just depends on whether she has access to the needed magic.
I’m most worried about Noxus getting the upper hand in its struggle against Demacia or Lissandra getting ahead in her Watchers’ summoning. We don’t really have an answer to either of those two things short of Ryze, Aurelion Sol, and Zoe breaking out the World Runes and nuking it.
In other words, the solution might be worse than the problem.
People will assume a lot about which is the worse thing compared to another, all based on conclusions that might not even come to pass and mostly generalized presumptions.
I remember when Mordekaiser was objectively the biggest threat, nowadays him and his kind are typically low on what is a very flexible list anyway. On another hand Aurelion Sol can be a "threat" but no one is willing to pay much mind as currently he's a dog on a leash. Only not a threat unless something comes to pass that draws him to destroy Runeterra, but that doesn't mean much because who says that will be an outcome! This is a similar analogy for pretty much all the other dangers to the world.
The closest thing where Runeterra was nearly destroyed came from the Rune Wars. If you want a definitive answer, my money is on that as the biggest danger because you dont need to read someone else's subjective opinion to learn how cataclysmic it was!
Riot.
Honestly with the revelation that the watchers are the masters of the void,I'd say that Lissandra is actually the most dangerous threat to runeterra,as she wants the watchers to return.
Though they are unlikely to try and destroy the planet,I mean they seemingly didn't try this in the past,so why would they try it now?
I'd say the Void wiith the watcherss....I guess because riot changed them from frozen beings to alien masters....
Noxus isn't going to destroy Runeterra at any point in time (except accidentally, but anyone could do that), so they're probably one of the lowest threats.
Lissandra is also probably on the same level, except wanting to kill mostly everyone, rather than subjugate them.
Darkin probably lack the necessary power to actually destroy Runeterra, although they might cause it to be ravaged by war completely.
The Void is probably the only faction that is actually seeking to destroy Runeterra itself. Everything else wants to kill and fight. They're the only ones who want to actually destroy.
Potentially, the Void.
At the moment, the Shadow Isles or Xerath.
Those toxic Yasuo/Riven mains having an all out fucking war.
Seriously(?) though, Ryze. He's going to get all the world runes then go all Thanos on our asses.
Honestly I hate how no one looks at the war for shurima, there littlery celestial beings who are going to kill each other for the power to rule us he a big land. The after math in that can be horrific. If xerath wins he makes slaves of azir wins he returns shurima to an empire that can be noxus and demacia.
It's not really that easy to decide since each region is dealing with something
The danger of the watchers coming back, starting from freljord obviously 
The shadow Isles and the black mist engulfing runeterra starting from Bilgewater 
The Void invasion that would definitely start in Icathia 
The only global threats there are are in very minor numbers like the 4 demons (
and the one 
is controlling) and the 5 Darkin (
and 
as Rhaast are the only 3 out of the 5 we know)
's people use a Moon stone to keep a giant threat at bay...
Void easily. But not the Voidborns. Void energy is a lot more dangerous than Voidborns. Voidborns seem defeatable, see the Ascended.
The Void and the Darkin Reunion. People keep feeding the void while 3 out of 5 Darkins are known to roam across Runnetera(except Rhaast), technically Varus' sister is on the loose too but her ststus is unknown. Swain is preparing for "the darkness that is coming" while Lissandra isn't ready yet. Who knows, maybe there's something even darker roaming by...
The Void and the Mist are the most obvious ones, since their goal both is to consume Runeterra basicly. The watchers, of course If Ryze fails to collect all his runes, that might end with a world destrying catalyst. With the amount of magic going on there, Ionia itself can be quite the danger just by existing
While Noxian have no borders to their methods, they mostly stay away from stuff that destroys themselves at the end Demacians with their hatred for magic might end up ending their world with their blind idealism, but for now there is they do not have the power
Darkin and other demons are unlikely to destro Runterra. Darkin are bound to certain limits as body possessing parasytes and the demons all have their one thing they do, all of them harmful, none of them likely to destroy Runeterra ever
