Unless Anlokar wrote more than one Rivellon destination in your book of coordinates, the portal would have to be back to the Raanaar world or one of the places that would be in a student's book. The hero could be going for help, with Damian intending to build a power base to try to get revenge on the Divine One (presumably in DD2), or to help the Raanaar rebuild, since he/she was part of the group responsible for Damian being there in the first place. If it wasn't just a portal to the after game battlefields, we may find out the destination in DD2.
I would assume story-wise the crystal remains in the Raanaar temple to protect that world from demons. That was how it was described in the dialog near the end of act 3, even though the crystal remained in your inventory when you moved to act 4.