If I understand correctly a unit that engages an enemy in terrain is considered in contact with the enemy units front edge no matter where on the terrain it is touching. (RR 81.8)

How would you treat Kari's Wraithstep in this circumstance? It doesn't allow her to flank an enemy since she will always count the enemy's front edge as the contact edge. I would also assume she can't use Wraithstep to move to another place along the terrains edge either but I can't find any rules about this. Thoughts?

In this instance she gets removed from the board until that unit leaves the terrain. Then she is placed in contact as required and joined by 500pts of chaos space marines.

In all seriousness though, wraithstep doesn't work because the unit in the terrain has no back edge. However, if Kari was in the terrain she could wraithstep out of the terrain.