We all know that Nimueh and Morgause were High Priestesses of the old religion with remarkable magical powers. In their time, they both wrecked havoc in Camelot beyond control and, if it weren't for Merlin, they would have succeeded in bringing the kingdom down from day one. However, it's obvious that at series one both Morgause and Nimueh were at least on an equal magical level with Merlin, if not a lot higher, since they were experienced in using magic both in combat and in spells. However, they ultimately failed, because Merlin killed Nimueh and fatally wounded Morgause, only for her to sacrifice herself and leave that plan to her half sister, Morgana.
Wouldn't it be A LOT better if Morgause and Nimueh teamed up? There is no way Merlin could defeat both of them, since he could hardly manage defeating one of them at a time. Since both of them loathed Uther Pendragon, why didn't they make an alliance in order to destroy him? There is no doubt that they would succeed in that. Could it be that they weren't aware of another high priestess left alive? I don't think so, because Nimueh was implied to be a seer and could see the future and also see Merlin through her crystal ball. Could it be that Morgause was away or in progress of becoming a high priestess? Maybe, but she seemed one of the strongest magical people to exist in season 2, which means that she was already at a pretty high level in season one. I don't think that the writers gave much thought on explaining that one. What do you think?