Contrary to popular belief, stars are not immortal, even in the world of Faerie. So although Yvaine outlived Tristran, and was still reigning in Stormhold when all who had seen the royal couple arrive were dead and dust, her time too came inevitably to an end.
The story of her end is still told, though none now live who saw it. They say that in her final days, she withdrew to the top of the fortress. One day, her servants saw a bolt of flame plunge from the tower, and when they broke down the door, she was gone.
They say the fort was surrounded by unicorns to mark her passing.