The Shade of the Sun

Ren always knew that Penny had a thing for mystical creatures and artifacts. In fact, she loved them, but all the dribs and drabs that Mrs Russell sells never works. Ren thought the onyx ring would turn out to be the same as all the rest, but it soon takes them to another world with the power of the Sun's Blessing.
Now, trapped in another, apocalyptic-looking world with absolutely no sun in sight, how can the duo ever hope to get home?