So I can't give them peace. OTOH, I can't give them the monsters from the other side. . .
Well, it's possible to fetch back some magical knowledge, to be sure, and make yourself a truly troublesome soul, but not enough to turn any land into one of the monstrous ones.
There's civil war, which leads to more complications than I want, since these lands are background. And the logical repercussions include disrupting their protection from the monstrous lands. Bringing a wayward vassal to heel is possible. So are bandits. But too much war would disrupt the whole world and the story with it. (Hmm. Only when poking around that does it spring to mind that it would also include more politics than would please me, and lean toward The Game of Thrones -- which I also don't want to do.)
But that means that the menaces have to be magics or monsters, but still minor enough that they are dwarfed by what lies in the other lands. (Though, come to think of it, they may not realize that, and may look down on the men who defend them from those perils on that ground. And ventur foolishly in search of glory.)
Hmmm. . . hmmm. . . . there might be lions. And bears. Possibly even dragons, though I'm not sure about that; dragons may belong only in monster-laden lands. Gryphons, on the other hand. . . or even giants. . . .
And wizards or sorcerers or whatever type of magic they have. They can do a lot of trouble. Much depends on the effects of their spells. Intrigue would be too subtle for the story, and what's more, too subtle to make knights or militia the solution. Your straightforward menace in a tower may lead to nice fighting, though.
Enough ideas, perhaps, for the story to play with.