This is a song called Lotus Blossom written by Billy Strayhorn, who was a longtime collaborator with Duke Ellington. He wrote some very famous songs, such as Take The A Train. Ellington was kind of an asshole and took quite a bit of credit for Strayhorn’s songs.
In 1967, Strayhorn died of esophageal cancer and Ellington recorded an album of his songs called “And his mother called him Bill.”
Ellington had recorded Lotus Blossom earlier in the sessions, but as the band was packing up (you can hear them chatting and packing up their instruments in the background of the song) he started playing Lotus Blossom solo and the engineer kept the tape rolling.
What was captured was a hugely egotistical asshole who frequently stole credit from collaborators playing his heart out on his own, when no one was really listening, to say goodbye to a dear friend.
I think it might just be the most beautiful thing ever recorded.