Alexis Petit-Will may have requested a “little” piece whose title ends with this adjective played with a setting of “O salutaris hostia.” That piece, whose title ends with this adjective, scored for 12 voices, two pianos, and a harmonium, was described by its composer as “the last of my péchés de vieillesse” (”peh-SHAY deh VEE-yay-esse”) after not writing an opera for 30 years. This non-year adjective nicknames a piece that quotes the hymn “O Lord, Save Thy People,” the (*) 1812 Overture. This adjective's Latin form names a Beethoven piece that ends with a fugue as voices sing “Et vitam venturi saeculi.” In Latin, this word describes a High Mass, or "Missa," that Beehoven dedicated to Archduke Rudolf. For 10 points, name this six-letter adjective that may denote a very serious piece, much like a type of vow. ■END■ (10[1])