Ombra a midlight of Umbra wishes to see a sunset... but how??? It just so happens that there is a Longray willing to teach him the way to stay in the other world long enough to fulfill his desires.(but what will the cost be for such a sight.)
(Tenebrae cannot return to Umbra fully for he no longer feels the pull back to that world but perhaps he can sway things in his favor and find the way back through another.)
Be warry.... the Eldars said for Tenebrae though not in this world can still cast down into Umbra through others and for a time use them to serve his purpose.