Who is Ma Yuan?

[ElMogwai]

I was reading the fiendish codex I and i read about him being a god slayer and trapped in a well of darkness but thats about all it says. is there any more information on him? which gods did he kill? are there any stories about him?

 

[Nagol]

Original Deities and Demigods (1e) has an entry for Ma Yuan in the Chinese mythos.

It is a monster that kills minor gods. It has an item that can be formed into any weapon it desires (including magical ability) -- a typical use if to create a replica of the weapon Ma Yuan is facing only twice as powerful. Its existence prevents complacency among the deities.

I have no idea about the original representation.

 

[Shemeska]

Those two sources are pretty much it. However I think there was a brief mention of Ma Yuan in a very late 3.5 print Dungeon article on the Wells of Darkness in the Abyss, perhaps a line or two. But that's off the top of my head, so don't quote me on it.