Step 1: Identify active voice.
The original sentence is active: Subject (king) + Verb (executed) + Object (bearded man’s brother).
Step 2: Recall passive structure.
Passive = Object + was/were + past participle + by + Subject.
Step 3: Transform.
Object: “The bearded man’s brother.”
Verb: executed → was executed.
Subject: the king → by the king.
Hence, the passive sentence: “The bearded man’s brother was executed by the king.”