Avant-garde drummer Milford Graves, one of the most radical of all jazz percussionists, and the unclassifiable pianist Don Pullen played a concert at Yale University in 1966, and Graves issued two LPs of the performance on his SRP (Self-Reliance Project) label. The first volume, In Concert at Yale University, is impossibly rare; I've never seen a copy. The second volume, Nommo, while still not a common record, went through several editions, and is easier to find. I have what seems to be the third pressing; the cover are has changed, and the equal billing of the two artists has shifted in Graves' favor. Later issues were on Graves' subsequent label, IPS (Institute of Percussive Studies).
The music was challenging for the time, and still is, frankly. These are atonal improvised duets, in free, unmeasured time - just about as "outside" as any jazz can be. But these are master musicians, and their sense of form imposes an order to these improvisations, even if the language is unusual, even "difficult." The YouTube video features the artwork from the first pressing.