If you don't kill Ishkabal (the first time) and immediately capture the Necropolis towns (this is possible, but rare, especially when playing on Veteran or Hero difficulty, but I was playing on Warrior), then after the first Lich is killed, a quest appears to capture these towns, and consequently, it will be impossible to complete this quest.
If you don't kill Ishkabal (the first time) and immediately capture the Necropolis towns (this is possible, but rare, especially when playing on Veteran or Hero difficulty, but I was playing on Warrior), then after the first Lich is killed, a quest appears to capture these towns, and consequently, it will be impossible to complete this quest.