The Rotten is an amalgamation of Undead bodies residing, and writhing, deep within the Black Gulch below the Gutter. The Rotten appears to have begun with an Undead trapped in one of the numerous iron cages found throughout Drangleic and is held together with chains. The Rotten embraces all, in his sanctuary for all things unwanted or tossed away.
The official strategy guide explains:
"A patchwork entity born from an amalgamation of lost souls and the withered bodies they inhabited, over time The Rotten absorbed so many life forces that he came to possess a Wonderous Souls of his own. He dwells deep below the surface world, embracing all that arrive at his sanctuary for things unwanted or thrown away and granting them new life as part of himself…"
There is one particular Undead on The Rotten's shoulder that mimics its movements, like it is controlling them.
Shalquoir mentions in passing that the Rotten has been at the bottom of the Gutter for a very, very long time and implies he may not have been rotten from the beginning.
When Shulva, the Sanctum City was enveloped in Sinh's poison, it may be that one of the fleeing survivors discovered the remnants of Nito's soul. This soul seems to have the perverse urge to gather power by combining itself with other less powerful beings. Nito was a conglomeration of skeletons, and this possibly explains one of the reasons behind his goal to spread death as much as possible (those who died can add their power to his). In contrast to this, the Rotten seems to take a more compassionate line, embracing all those who are unwanted or tossed away. When Shulva sank, he may have sheltered many who were fleeing in the 'Sanctum of the Scorned' (Unused content name for the Black Gulch primal bonfire). We know that the Pit has been used to toss away anything unwanted, and that the Rotten has been down there a long time, so this could mean that while the Rotten's origins may have had something to do with the fall of Shulva, its power grew over the centuries as figures like Lord Aldia and King Vendrick used the Pit to toss away anyone or anything deemed unwanted. The Rotten embraced these poor souls after…
Some of the more determined souls exiled
to the Gutter sought to scale the walls of the
forsaken place and walk the earth once again,
but failed, and wasted away mired in misery and
The Rotten's chamber contains a vast number of the poison spitting statues, with the curious detail that they don't actually spit poison. From outside the boss fog, we hear what seems to be a number of voices weeping. When we enter the boss chamber, the Rotten places a broken head on the body of one of the statues. When it falls off, the Rotten has a paroxysm of what appears to be rage and then attacks us. It seems from this that the Rotten has some strong connection to these statues, and perhaps whoever the Rotten was, once constructed them. The Rotten is also capable of an oil spitting attack. Its chamber, as well as the Black Gulch are full of small puddles of oil. From this it is implied that the Rotten is the creator of the oil puddles, and that the Black Gulch, while a natural formation, has been transformed into the Rotten's domain.