Few of us have access to a regular accompanist or orchestra. The MusicSolo series composed by John Roberts-James offers concert virtuoso encores or lollipops for solo, unaccompanied instruments. Some of the pieces are easy, most are difficult, and many involve <multiphonics> with the performer accompanying himself.
The French Horn series include 'From Opus 3' containing some 40 items, mostly difficult,
Five Multiphonic Sonatas for Unaccompanied French Horn Op 17
and - particularly challenging - the great Variations on a Theme of Paganini Op 19.