She is a Vanir, and she can foresee things, perceiving the truth before it becomes known, dreaming it in her sleep.

The death of her husband comes to her in a nightmare. This is no mere premonition. It has already happened and it hits Sigyn like an implosion of the worlds, absolutely and unforgivingly. The enemy is defeated and Loki is dead, beyond her reach, and Sigyn can do nothing but scream, her heart tearing itself into bleeding pieces with every drop of a tear. 

“Simply live, endure, until you feel alive again,” someone tells her later and she wonders why they should care, since it was Loki who was her husband once. They must be glad that he is gone.

But she ponders on their words, and they hold a twisted meaning to her mangled spirit. She will live, but she will never feel alive again. So Sigyn, exhaustion and heartbreak overwhelming her, whispers an ancient spell, dangerous and terribly dark, and she never awakens again, breathing, but not truly alive, forever dreaming, of meadows and entwined fingers, of happy whispers and shared kisses, from so long ago. 

There, she reclaims everything she has lost, and finds no reasons at all to shun the illusion and return to the world. Sigyn has given herself millenia of perfect happiness.