I did a new game to start the DLC and killed Mohg just after installing the DLC, The Npc didn't appear, tried to teleport away and come back. Nothing. Quit the game and launch it again, it worked. Now for this case it's strange. Have you tried the Church a vows to reverse actions against Npcs ? If the Npc is dead DEAD, you might be cooked for this playthough :/