It is an abomination, made from corpses held together by magic, chains and, possibly, the soul of Gravelord Nito. As of Dark Souls lore the Rotten is made from essentially garbage, things that the locals of Majula have thrown away.
Unnatural Strength Edit
The Rotten uses a large chunk of metal as a cleaver to split intruders in twane. It also grabs up the player character, trying to grind up their bones into nothingness with it's bare hands.
Getting the cleaver and reading it's description indicates that the Rotten consumes anything that falls into Black Gulch from Majula, which means he may eat the player's corpse upon death.
Hex Casting Edit
The Rotten is a living hex catalyst, spewing forth mass quantities of the material hexes are made of, As well as imbuing the material into the blade.
Fire Resistance Edit
The Rotten appears to be completely immune to the molten puddles that surround it. he also is barely affected by weapons that deal fie damage and even pyromancies such as forbidden sun and great chaos fireball.
Chopping Off a Limb Edit
If the player hits the Rotten's cleaver arm (the arm that holds the cleaver) enough times, it will simply fall off, leaving it with one arm.
Attacks From the Rear Edit
The Rotten is a very slow and cumbersome creature, so hitting it in the back seems to fluster it.