Here he is, the man himself!
Thoughts... yes, it says, "killer of the gods". But then, in 1E, it was a heck of a lot easier to kill gods.

I'd say he might be powerful enough to take out some DvR1 or even 2 demigods, but only after a tough fight (or if the god is unusually weak), though he shouldn't have as much of a challenge with avatars. He has 300 hp, which is more than Cerberus, but less than most of the Titans in the same book, so I would make his CR between the two groups, like 22-25.
MA YUAN (killer of the gods)
FREQUENCY: Unique
NO. APPEARING: 1
ARMOR CLASS: -4
MOVE: 24"/24"/ /24"
HIT DICE/POINTS: 300 hp
% IN LAIR: 10%
TREASURE TYPE: A (X 5)
NO. OF ATTACKS: 5
DAMAGE/ATTACK: 3-30 (X 4)/5-40
SPECIAL ATTACKS: See below
SPECIAL DEFENSES: +3 or better weapon to hit
MAGIC RESISTANCE: 65%
INTELLIGENCE: High
ALIGNMENT: Chaotic evil
SIZE: L (70')
PSIONIC ABILITY: Nil
Attack/Defense Modes: Nil
LEVEL/X.P. VALUE: X/32,300
This monster has 3 eyes in his tyrannosaurus-shaped head, and has 4 large humanoid arms. He has the strength of a storm giant, but rarely relies on this in battle. He also has a powerful magical device shaped in the form of a small triangular piece of stone that has the power to turn into any weapon the holder wishes, magical or otherwise. This monster's favorite tactic is to make a weapon from his stone that is double the power of the one being used against him. The creature is known to have killed at least 10 minor deities. It attacks as a 16+ hit dice monster, The existence of Ma Yuan prevents complacency among the gods.
some preliminary stats for Ma Yuan:
Ma Yuan (killer of the gods)
Colossal Outsider (Chaotic, Evil, Extraplanar)
Hit Dice: Xd8+X (X hp)
Initiative: +X
Speed: 60 ft (12 squares), fly 60 ft (good?), swim 60 ft
Armor Class: X (-8 size, +X Dex, +X natural), touch X, flat-footed X
Base Attack/Grapple: +X/+X
Attack:
Full Attack: 4 (arms) +X melee (X+X) and bite +X melee (X+X) (depicted carrying a sword)
Space/Reach: 30 ft/30 ft
Special Attacks: X
Special Qualities: damage reduction X/epic, darkvision 60 ft, spell resistance X
Saves: Fort +X, Ref +X, Will +X
Abilities: Str 39+, Dex X, Con X, Int X, Wis X, Cha X
Skills:
Feats:
Environment: (Any evil-aligned plane?)
Organization: Solitary
Challenge Rating: X
Treasure: X standard
Alignment: Always chaotic evil
Advancement: ---
Level Adjustment: ---
Ma Yuan is 70 feet tall, and weighs X tons.
COMBAT
Ma Yuan?s natural weapons, as well as any weapons he wields, are treated as chaotic-aligned, evil-aligned, and epic for the purpose of overcoming damage reduction.
Originally found in the first edition Deities and Demigods (1980, James M. Ward and Robert J. Kuntz).