I'd like to suggest that you give Deorcnysse a permanent darkling garrison to deter the AI from attacking it before it has the forces to defend it. With the current situation of the city being completely undefended, what will often happen is that the AI will walk right in with a small scouting force. A couple turns later hordes of angry Darkling armies swarm the city and take it back from the invaders. To show them who's boss, the darklings then raze their great mythical city to the ground in a fit of pique. It's not a very satisfying outcome and it happens very frequently.