(Author’s note: The following article cannot be considered the official doctrine of either Advanced Dungeons and Dragons or the Roman Catholic Church. However, it is compatible with AD&D, and except for the parts about Asmodeus it is not in conflict with works on demonology as generally accepted by Catholic exorcists, thus enjoying tacit approval by the Church. However, this article does not have a nihil obstat; much of it is original, and it approaches the subject from a different angle than a religious tract would and should not be considered as such. The rise of Asmodeus is not documented in any major text on demonology, but very little original work on the subject has been done since the Middle Ages, so it is possible that the situation has changed. Perhaps Mr. Gygax has more accurate sources of information...)
The first half of the article is a mythology, to my eye a pastiche of Biblical and Dantean ideas, and twice referencing the Lesser Key of Solomon. This presents a historical tale explaining the overthrow of Satan by Beelzebub/Baalzebul, and then likewise by Asmodeus, in such a way as to explain why it is that latter figure who leads the Hells in official AD&D rules. It identifies the first coup with the real-world Renaissance period, and the second with the period of World War II. Von Thorn writes:
In fact, most people including the majority of the clergy, were not aware of the change in the leadership of Hell. To this day, when people think of “The Devil,” they think of Satan, and if the name of any devil is mentioned in a Catholic sacramental ritual, it is his (e.g. when the celebrant asks, “Do you reject Satan?,” etc.).
The latter half of the article presents information on three now-fugitive figures: Satan, Belial, and Astoroth (i.e., famous devils who had not previously appeared in works of AD&D). Here is the stat block and the first part of that ten-paragraph section:
A few details are given that would certainly be proscribed later on, such as: a suggestion that an actual paper contract between Satan and an interested PC be drawn up for clarity; special status for clerics of mainstream Abrahamic religions, etc.:
It is best to have the contract actually written out, with identical copies for the player character and the DM (which the player should study carefully before signing), because Satan is extremely literal, and he’ takes sadistic pleasure in twisting the intent of a contract by fulfilling its letter...
Naturally, a person cannot take any positive action to get out of a contract without first deciding to do so. Any Jewish, Christian or Muslim cleric of at least Patriarch level (such as a Cardinal, Primate, Metropolitan, Chief Rabbi, Ayatollah, Caliph or equivalent) can invoke the name of the Deity to save the person’s soul. However, they usually require some token of one’s devotion to the cause of good, such as the performance of some appropriate Quest, as the price of redemption...
It's likely that articles like this one by von Thorn -- including those by Ed Greenwood in Dragon #75, 76, and 91 -- touched a little bit too close to home in some circles, and were among the catalysts for the witch-hunts regarding D&D in the 1980's. You can see some of those cultural warriors of the period (notably Pat Robertson) still making the same warnings about D&D today. In order to survive as a business, TSR made references like the above completely prohibited from their publications, a taboo that survives to this day for owner of the official D&D license.