So, the short of it, the One is the Shezzar(ine). Probably.
The One has nothing to do with Lorkhan and little in common with him. The Alessian Order paid no dues to the Eight or Shezarr; they supplanted them.
This monotheistic religion was once very popular, but today only remnants of its faith remain. It started in the coastal jungle of what is now the Colovian west, where a prophet named Marukh, who had spoken to the "Enlightened One," Saint Alessia, began to question the validity of Elven rule. These sentiments led to an increasingly abstract and unknowable depiction of a Single God. The Alessians were wise enough to realize that they had to incorporate the ancient polytheistic elements into their new religion for it to find a wide acceptance. The divine aspects worshipped by the various humans and Aldmeri were recognizable in the guise of the myriad saints and spirits of the evolving Alessian canon. It wasn't long before the Order was the authority on every religion in Tamriel, and their power grew to earthshaking proportions. Nearly a third of the First Era passed under their theocratic rule. When its priesthood had become too widespread to support itself, the Order began to fight among itself. With the severance of the territories of West Cyrodiil from the Empire, too much money and land had been lost. The War of Righteousness broke out, and the Order which had almost ruled the world undid itself in a ten year span.
Monotheism is nearly impossible in Tamriel, where so many deities obviously exist. Compare the One to the Skaal's All-Maker or the Yokudan Satakal. It's something of a synthesis, or a putting-back-together of parts that were once whole.