You deactivate Stendarr’s ward using the Ring of Stendarr’s Mercy before opening the gate to fight Mannimarco, making it possible for Molag Bal to intervene. As to why Molag Bal picked up Mannimarco and no-one else, it’s possible that he could only take Mannimarco because he was dead, and therefore weakened. At that point Molag Bal had also realized that Mannimarco was a much bigger threat to him than the Vestige, so maybe he just forgot about the Vestige in that moment.

LittlePinkDot wrote: »

Was Mannimarco dead? I thought he was still alive at that time. I know the Vestige is technically dead.
Also at the end of the game Bal seems to suggest that he knew he was going to lose when he said the elder scrolls were fulfilled. Then said there were worse masters than him. If he knew he was going to lose, then why even bother with the whole plane meld thing in the first place? It seems like hes scheming something else.

You kill Mannimarco in that quest (although it is possible that he had died a first time before that). Molag Bal probably didn’t know that he was going to lose from the beginning, but probably realized after the Planar Vortex was destroyed.

VaranisArano wrote: »

Mannimarco dies at the end of your fight with him and promptly begins to gloat about you only think you defeated him. Actually, he's become a lich and you've only completed his transformation. Soon, he will be greater and more powerful than before and will crush you like a bug...wait, was that...Molag Bal...master, I swear I didn't mean it about betraying you - AUGHHH!

He actually become more powerful but when get Mantella on hands and achieve godhood at ending of Daggerfall, still Jills of Akatosh try repair this and split him to feeble mortal whos found his dead on hands of Champion of Cyrodiil and godlike Revenant who eclipse the Arkay