As far as we know the void is the immediate threat to all life on runeterra, it's the main plot line in the lore, but if the void is the main villain here then what about the shadow isles are they somewhat connected to the void? or is it a whole different evil with defferent purpose? , or the void considered the outside threat and the shadow isles are the inside one? what if 
decided to lead the undead army to runeterra (night king style) ?
I'm very certain the Void and the Shadow Isles are two separate threats that don't have anything to do with each other. And out of those two, the Void is the bigger threat. Because yeah, the undead is bad, but what is the undead against something that literally seeks to destroy all of existence and existing? Then death would be rendered completely useless.
Perhaps it might actually be such a big threat that the Shadow Isles switches priorities and starting trying to hold off the Void. Since in this instance, the antithesis of life is not death, but is instead non-existence or absolute nothingness, since as they prove to us, life still exists after death.
Also, special mention to the darkin, who while not such a looming threat like the Void, do kinda have the same goal as them. At least that is in Aatrox's case, who wants to destroy Runeterra in order to destroy himself and end his own misery.
They are separated.
I don't think the void was ever called the main-plot of the lore, thats just what everyone started to assume based on the fact that it is easily the biggest threat in the lore.
The shadow isles have no known connection to the void and while the void seeks to erase all existence(not just runeterra but the whole universe), the shadow isles seek to spread their curse and turn everyone into an undead.(though Mordekaiser himself likely just wants to conquer runeterra, it probably doesn't matter to him if they are alive or undead by that point).
The real question is what's a bigger threat? The Darkstar of The Void?
The shadow isles being isolated does A LOT to mitigate it's threat. The void threat being pretty central in Valoran means it's threat is immediate.
is another "world ending" threat.... And she affects an area a little smaller then the shadow isles (counting the water all the way to Bilgewater)
