The Shatterfield

The realm answered a knock. Now it has to survive the door.
One winter after the storms began walking the eastern water, Elowen and Caelan do the one thing four hundred years of history was built to prevent: they sail east, past the wall, to find whoever has been calling. What they find is not a weapon. It is the people the realm's own founders tried to erase - the Shadow line, exiled into a wasteland of glass, still alive, still holding a wall of their own against something far worse behind them.
And the glass is not neutral ground. It raises Elowen's power to something she has never been, and terrifies her how much it feels like home. It strips Caelan of his Steel entirely, leaving the realm's most powerful mage powerless on the exact ground his family shattered. And it wakes the Shadow in Aric all the way - the returned son, walking home to kin he never knew, to a wall his own oath swore him to keep them behind.
Three bloodlines, standing together on the glass for the first time in four centuries. The exact configuration the founders killed a whole court to prevent.
Because the thing in the deep of that broken country has been waiting four hundred years for the number to be three again - and it did not lure them across a sea to be stopped.
To save a people, a marriage, and a world, the three of them must answer the oldest question the realm ever buried: what does three do - and whose hand turns the key.
Salvation or ruin. The same key, turned the same way. The difference is in the hand.