Learn Farsi: سرنوشت این قهرمان مجهول است.

It follows the ma-C1-C2-u-C3 (Maf'ul) pattern. The three-letter Arabic root is ج-ه-ل (j-h-l), which has to do with ignorance or lack of knowledge. Plugged into the pattern, it becomes مجهول (majhul), literally meaning "that which is not known" or "unknown."

What is the related active form of مجهول, and what does it mean?

The active participle pattern for Arabic loanwords is C1-aa-C2-e-C3 (Fael). Using the same ج-ه-ل root, we get جاهل (jahel). While مجهول is the passive "unknown" (the thing not known), جاهل is the active "ignorant" (the person who does not know).

The word سرنوشت (fate) looks like a combination of two familiar Farsi words. Is it?

Yes, it is a compound of سر (head) and نوشت (the past stem of نوشتن, to write). It literally translates to "written on the head," linking to the cultural concept that a person's destiny is written on their forehead.

I hear an ezāfe in سرنوشت این قهرمان (sarnevesht-e in qahremaan). Don't we usually drop the ezāfe before words like این (this)?

You do not use an ezāfe when این comes before the noun it points to, which is why there is no ezāfe between این and قهرمان (this hero). However, the ezāfe on سرنوشتِ (sarnevesht-e) is necessary because it links "fate" to the entire possessor phrase "this hero" (the fate this hero).