A Passacaglia is an old Spanish or Italian dance. Musically, it is associated with variations on a ground bass, usually relatively slowly and in triple time. There are a number of famous examples in the organ repertoire, probably most notably the great Passacaglia and Fugue in C Minor by J S Bach.
Passachordlia is a series of variations for Organ, loosely based on this structure, but instead of the ground bass being the sole unifying factor, instead the variations are actually based on the harmonies derived from four scales, shown below. Each scale is used for a quarter of each variation, always in the same order.
The first ten variations grow organically out of one another, before a seeming complete change for variation eleven. However, despite this, the scales are still the same and the original ground bass can be found hidden from time to time before its triumphant return in variation twenty. From here it is full pelt to the finish, where, in the final variation, each scale is used to build the enormous chords from which the title was derived.