Complex sentence: He stumbles upon a beheaded man of the Night's Watch, and is attacked by a Wight, who is killed by the Lord Commander Jeor Mormont (James Cosmo), before being reprimanded for not sending Ravens south to warn the wall of the incoming army.
Split into simpler sentences: He stumbles upon a beheaded man of the Night's Watch, and is attacked by a Wight, who is killed by the Lord Commander Jeor Mormont (James Cosmo). The Lord Commander reprimands Samwell for not sending Ravens south to warn the wall of the incoming army.