Help with Homework

Dear readers, since most of you are more well-read than I, please let me turn to your gathered knowledge for advice.

This line is from Chapter Two of Volume VII of my as-yet unpublished Starquest space epic.

Yarrow the Otter said, “Lord Ahab is King of Pirates. After this raid, he shall be called Overlord. After this raid, he shall be called Magister Militiae Caelestis, the Prince of the Heavenly Host!”

 

Athos heard a hiss of detestation from Mantis. “Blasphemy!” the Blue Man muttered.

Is the declension correct?

I know that Yarrow’s translation is wrong, since he is saying “prince” rather than “master” — but I do not know the word ending.

I believe the chief of the legions of Rome (the City of Man) back in the day, was called the Magister Militum and I know Michael the Archangel (whose role in the City of God is like this) is addressed as Princeps caelestis militiae. 

Magister Militiae Caelestis?

Magister Militum Caelestum?

Something else?