Is there an example of a god changing alignment in the published lore of D&D settings?
I'm brainstorming for a future campaign and want it to revolve around a Good or Neutral god of knowledge who has turned Evil. I'm not sure if that's even a plausible premise though, hence the question.
Yes.
In the original Forgotten Realms setting, the supplement Old Empires by Scott Bennie detailed the realms of Mulhorand, Unther, and Chessentea. The Untheric pantheon, in particular, was ruled by Gilgeam:
Enlil himself retired; he appointed his son, Gilgeam, as his successor as king of the gods. At first, Gilgeam was a just ruler. But sometimes even the gods can go mad.
Gilgeam was a proud god-king. As the centuries passed and Unther declined in prosperity and influence, as taxes rose and the peoples hatred grew, Gilgeam became a cruel and jealous lord.
Gilgeam's alignment in stats blocks is given as Lawful Evil, but the text described above makes it clear that this was not always the case, unless one can reconcile "just ruler" with a lawful evil alignment. Further, it is quite clear from the text of the book that Gilgeam was not simply a mortal proclaimed as god-king, but a god (indeed, the chief of pantheon) in his own right, with defined major and minor spheres and leveled clerics described in the supplement.
Here are four examples from the Forgotten Realms, with clear in-game stories explaining the cause of the changes:
Mystra, the goddess of magic, has had multiple incarnations. In her initial incarnation, she was called Mystryl and was CN. After her first "death" due to the folly of a mortal mage (Karsus), she reincarnated herself as Mystra and became LN. While Mystryl was about the endless possibilities of magic, Mystra was about structure and maintaining order. Later, during the Time of Troubles, she got reincarnated again as Midnight/Mystra and turned NG. While earlier Mystra was not interested in what purpose magic was used, Midnight encouraged its use to make Faerun a better place. (The fourth incarnation is still NG.)
Amaunator, the solar deity of law and order, used to be LN. After losing too many worshippers, he died (or became dormant). When he got reincarnated centuries later, he was briefly LG (4th edition), as his followers believed that he had merged with/he was also the God of Dawn, Lathander (a NG deity). Following the event known as the Second Sundering, he came back to his old self, separated from Lathander and turned LN again (5th edition).
Jergal, the old Lord of the Dead, used to be LE. Then he relinquished most of his portfolio (which included undeath and tyranny) to Bane, Bhaal and Myrkul; and turned LN.
Mask, the god of thieves, was NE since his very introduction in Dragon magazine #54 (October 1981) until the end of the 3.5e era. A number of novels have revealed that the original source of his divine essense is the goddess Shar, who is also NE. He died and disappeared for the 4e. When he made a comeback a century later thanks to the efforts of his chosen, he became CN (5th edition).
Here are two more examples, but without any specific and well-established in-game explanations:
Tempus, the god of war, used to be CN in all editions of the game before the 4th. During the 4e, he became unaligned (as the alignment system was simplified). Strangely, when the 5e brought back the original alignment system, he became N, not CN. We can speculate that the formalization of a code of conduct called "Tempus's Honor" amongst his followers is relevant; perhaps Tempus started to care more about something like real-life Geneva Conventions.
Siamorphe, the demigoddess of nobility, was LN in all editions of the game until the 4th, during which she became LG. She is not listed amongst the deities of the Faerunian pantheon in the 5e Sword Coast Adventurer's Guide (SCAG), so we do not know what her current alignment is. However in SCAG, it is now stated the s/he is incarnated as a different noble mortal in each generation, so we could take this as the retconned reason for the change in her/his alignment.
If you want more details, there are a lot of FR sourcebooks, as well as nicely written summaries on the web.
I am by no means an expert on the lore about the Forgotten Realms, but having the Sword Coast Adventurers Guide I can at least give some educated feedback:
As far as I know there were the so called "Tablets of Fate" which named each deities portfolio. So the task/job of the god is more or less hard-wired but it doesn't say so about its alignment, also it also does not affect every deity, only those of the "material plane". So as long as his actions do not go beyond his portfolio, I guess he would be free to do as he wishes.
Either way you must consider that some of those gods are very very old and had quite the time to think about everything. To change the alignment of such an old and wise being there has to be some really immense event.
According to the SCAG there are cults of foreign gods and local divinities. You could also think about something like a demi-god. There is nothing in your way to prevent you from creating a god of your liking. He's not bound to anything of the official lore if you don't want him to.
You also wouldn't need to research into the background of the god or conflict the official lore.
Well, the lord moves in mysterious ways. Of course you could lay out your campaign that the happenings are well put together by the deity and everything is just as it planned it to be.
In the end you are the DM and you decide how the deities act and whether it is normal. Maybe in your variant of the Forgotten Realms the gods are pretty young (The Time of Troubles was like 150 years ago) or they need to find themselves anew. You decide!
