Here, I just wanted to make the main forces that the Darkin might have faced during "The Great Darkin War" (Some of them are canon while others are just my guesses)
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Janna, Vladimir, Zaahen, The Warrior, Myisha, and Ta'anari fought, betrayed or stopped some Darkins as we know it. (Also there are Nasus and maybe a few more darkins too)
During the Great Darkin War, the Darkins were all over Runeterra so I wonder if they faced some powers that made them retreat? For example, Janna defended Osha Va'Zaun during this war. We know that.
Perhaps in the Freljord, the Darkin faced off against the Demigods of the Freljord. Especially some powerhouses like Volibear or Anivia.
In Bilgewater, we have Nagakabouros. This strong spirit-god could have an important role during this war in defending Bilgewater.
In Ixtal the forest itself is hard to pass for mortals but I wouldn't be surprised if the Darkin could pass through it.
And maybe this is where Skarner might have intervened? I think Skarner is also strong enough to fight back against the Darkin.
And for the Blessed Islands, the magical mist might have stopped the Darkin from coming here.