Morgathism can apparently be traced back to the Mafani empire, although little is known about the specifics of its ancient history. What can be stated with some certainty is that there are many old cults scattered around the Mafani gulf and the Venarian Sea which have elements in common with Morgathism, although few of these cults are really significant in political terms these days (except in Dalkesh, where the modern state religion has many elements in common with the Meokolis brand of Morgathism). There are two modern versions of Morgathism (using that name for their god), namely the Meokolis orthodoxy and the Golothan heresy.
According to Meokolis Morgathism, there exists a multitude of gods, and indeed it allows for the existence of any and all gods. However, Morgath is believed to be ultimately the greatest of all, since He is Death (not just a god of Death, but Death personified), and all things must die, even gods. So all things ultimately belong to Morgath, and will be brought under his eternal dominion in the end, like it or not. It is not exactly a bright and cheerful faith, but neither is it (overtly) evil; it is characterized by a high degree of personal mysticism, whereby the believer attempts to acheive spiritual unity with Morgath before shuffling off this mortal coil. Suicide is not encouraged, as Morgath will get what's his when the time comes anyway, but many of the more "casual" believers do tend to adopt an "eat, drink, and be merry, for tomorrow we die" attitude (and in fact, many of Morgath's lay followers also attend other temples, in particular Halea's; Morgath doesn't mind, although some of the other temples might not let them in).
This whole business about Bukrai and chaos and evil is, according to Morgath's priests, a smear campaign orchestrated by Triadists or Agrikans in order to weaken Morgath's temple politically; the official stance on Amorvrin and Gulmorvrin is that, although Morgath of course may do as he pleases with the dead, any horror stories involving the walking dead have their basis in mortal sorcerers dabbling in necromancy, which is sacrilege and strictly forbidden, and is one of the few temple crimes to carry a death penalty. Some critics (particularly newcomers to Meokolis from the northern lands) accuse these Morgathians of being lying necromantic scum who worship pure evil in secret; such critics seldom make their stay in Meokolis an extended one, since the Emperor himself is a Morgathian and the critic has thereby made himself guilty of blasphemy and slandering His Imperial Majesty, and will be lucky to escape alive.