solemn

dignified and somber in manner or character and committed to keeping promises; "a grave God-fearing man"; "a quiet sedate nature"; "as sober as a judge"; "a solemn promise"; "the judge was solemn as he pronounced sentence"

characterized by a firm and humorless belief in the validity of your opinions; "both sides were deeply in earnest, even passionate"; "an entirely sincere and cruel tyrant"; "a film with a solemn social message"