So Mannimarco searched for another way, sacrifice own Necromancer to make him double agent, so he can achieve trust of Sai Sahan.

Just imagine, Mannimarco need Amulet of Kings, but Sai Sahan is too sturdy in his willpower even Duchess of Anguish can't break him, Nontheless he will escape somehow eventuallyДЄs Necromancer you actually can be agent of Mannimarco, Hail Black Worm! I bet he lost a lot of power for a short duration when he was wounded so there might be a window for escaping. To the question of the topic: I assume he escaped at the point when we did weaken molag bal in the main quest when we defeat him. The plan he has as actually not bad because of his superior knowledge about soul gems he knew that the amulet of kings is a soulgem which has the power to prison daedric gods which is crazy and only 5 of the mightiest persons of this time could stop him and presumably he would have killed them if molag bal did not intervene.

He always tried to achieve godhood which he actually did in daggerfall then so i assume it was a attempt from his younger years. I don't know why they made him so dumb in ESO but when you see how he acts in Daggerfall, a cunning influential Necromancer operating hidden, you might think what happenend in ESO then. Except for the weak depiction in ESO and maybe Oblivion(which is up to debate if it is the real one) Mannimarco is actually a very strong magician who survived for couple thousand years now.

Oblivion king of worms