The Basset Horn is an alto Clarinet (similar to the modern Alto Clarinet) transposed a 4th lower than a Bb Clarinet and has an extended range to low C like a modern Bass Clarinet.
Mozart used it in operas like “Die Entführung aus dem Serail”, “La clemenza di Tito” and “Die Zauberflöte” as well as in the wind serenade “Gran Partita”, the “Maurerische Trauermusik” and some chamber music works. Even the famous Clarinet concerto was planned for a Basset Horn in G and was eventually “released” for a Basset Clarinet in A (also with the extended 4 lower halftones).
After Mozart it fell into oblivion; Mendelssohn and Weber have written individual pieces, and then Richard Strauss used it in his operas and wind serenades.
Mozart (aka Süßmayr) has never written the Basset Horn below the Bb Clarinet range, everything can be played with a Clarinet sound.
NotePerformer contains a Basset Horn patch, in my Cantai first steps I used the VSL Studio Basset Horn.