Cuius is the noun form of the verb (a gerund?) 'who is'. The sentence isn't in a question form.
Literaly - The who is the servant of Savus. The sense of it is - That unknown person is the servant of Savus.
5 years of it against my will. Though it really did help with my English and I had a knack for phrases like 'quid pro quo'.
If I am wrong - never mind. Isn't Jespah a lawyer?