At the close of the Age of Fire, as the world ends, all the lands converge upon themselves. The Heap is considered the furthest end of the world, where various remains from different historical periods have accumulated over time.
The Heap can accessed by traveling to the Kiln of the First Flame, and finding a bonfire there. There is also a bonfire in the Painted World of Ariandel, in the chapel by Sister Friede.
The Earthen Peak Ruins are a subsection of the Heap.