Fallen angel

For the sin of falling in love with a mortal she was forever banished from the light to guard the broken gate in the abyss.
An angel's heart is a powerful and greatly feared weapon. Each shot from her cannon drags a shard of her heart with it to pierce any that would force entry to the land of the dead and poison their blood with holy light. As more and more of her heart is torn away, soon all that is left is a black hollow. Incapable of feeling, the love for which she sacrificed everything is but a forgotten dream.

And so is born the fool at the gate.