http://www.terrariaotherworld.com/
Not too much has come out, but so far the synopsis is this:
"Terraria: Otherworld places the player in a life-and-death struggle to restore a once-pristine world – now overrun by a malevolent force that has corrupted nature itself – to its original splendor.
Combining elements of sandbox gameplay with role-playing and strategy elements, Terraria: Otherworld explores the Terraria experience in an innovative and unique way - providing a fresh approach to the “open world” genre."
And we also have a trailer.