This instrumental is the title track of Pink Floyd's second album and also features on the live LP from "Ummagumma" and on "Pink Floyd at Pompeii" film. That's to say this actually is one of the most important tracks in the band's early live tracklists. It is interesting to note that this song was split in four parts on "Ummagumma", each one bearing its own title (Something Else, Syncopated Pandemonium, Storm Signal, Celestial Voices).
I think this cover art actually depicts the title song.
Roger Waters explained in an interview that the track was a musical description of a battle and its gruesome aftermath, so that the first part describes the start of the conflict, the second section is the battle itself, then we have the deadly aftermath and the subsequent mourning. For sure, this is a very deep and lysergic track, full of dark atmospheres and unpredictable contrasts, an experimental piece of music that's one of the most tricky, obscure and intriguing songs by Pink Floyd.