This sentence is from Death Note, specifically on how to use it.
This sentence is written in-universe using English. It states, “This note will not take effect unless the writer has the person’s face in their mind when writing his/her (the victim’s) name.”
I understand that 書く is modifying 人物, which would lead me to believe that 人物 is in reference to the person writing.
However, 人物 being the owner of 顔 would imply that 人物 is the person whose name is being written.
What is going on, grammatically speaking? Is this sentence ambiguous to native speakers? Would it be any more or less ambiguous without the story context of Death Note?