The Key, and the Binding

Therein lies the key,
to what ties you and me,
It came on the needed day,
So neither two would fray;
under the weight of the dark
in these the corners so stark.
My sister, my friend, my Love.
A blessing from the gods above,
We are now bound at the hearts,
and Hell can fall apart;
as wretched and undone as we be,
the ties that now bind, set us free.