"Loki was condemned by Skadi, the goddess of winter, to be imprisoned deep within the earth. A venemous snake was put above him, dripping its venom on Loki's body as an eternal punishment. Sigyn, his wife, begged Skadi to let her catch the venom and Skadi agreed. Sigyn holds a bowl beneath the serpent's fangs, gathering the poison. But when the bowl fills, and Sigyn goes to empty it, the venom fron the snake's fangs drop on Loki's body. This causes him to writhe in pain so violently that it causes earchquakes. It is said that Loki will be freed from this torture on the day of Ragnarok."
Or something like that.
My other Loki and Sigyn fanarts are here: [link] [link]
This is where she's afraid to leave him because she knows he'll get hurt.
I know it's not quite the real story, but I like romanticising stuff.