Complex sentence: Brian hears him and tries to hide under Peter and Lois's bed, but Quagmire finds him and beats him severely before warning him to stay away from his house, otherwise, "I'll blow your head off!".
Split into simpler sentences: Brian hears him and tries to hide under Peter and Lois's bed. Unfortunately, Quagmire finds him and punish him then before warned him to staying from his house, but otherwise, "I'll blow your head off!".