In Forma Hominis
In Forma Hominis is a prepositional phrase modifying diabolus. In is the flag for the prepositional phrase, and takes an ablative in this case. It means in.
Forma, because it is the object of the preposition, takes the ablative, and so is an ablative singular feminine type one noun. It means form or shape.
Hominis (hominis,-is) is a type three masculine singular genitive, because it modifies forma. It means man.
Thus, you get: in the form of a man
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