The Rat King is an ancient ruler of the underworld and the leader of the Rat King Covenant. Long ago, he struck an accord with a human chieftain; it was agreed that humans would rule the lands lapped by the sun’s rays, and the rats, all that was below. But the time came when the humans broke their word. They coveted the riches of the rat’s realm and in their greed they hunted the rats down. Tales of the Rat King’s wealth endure to this day, and we are told by Laddersmith Gilligan that if the Rat King is as rich as they say, he might have to pay him a visit. We can only imagine what the Rat King would think of such a character and his transparent greed. It is likely the rats were trying to stop the humans delving deeper into their realm, and so the human’s fought back. For this betrayal, the Rat King never forgave humankind and now sees them all as liars, schemers and cheats. But the Rat King is a noble and merciful being, he understands that honor matters more than gain, and therefore he pities the humans, for they are blind and foolish. Even though the humans broke their accord, the Rat King continued to honor it, and so he remains below ground, only seeking to defend his burrows and realm from any human who would trespass.
The Rat King is not alone in his plight, there are many who have come to the aid of this eminently noble being, and this is how the Rat King Covenant was established. Fellow rats, dog rats, mastodon warriors and perhaps even the Gyrm all stand to test one who would seek audience with the Rat King. The Rat King even put in place a trial for the brave to face should they attempt to reach him in the Doors of Pharros or the Grave of Saints. The Royal Rat Authority and Royal Rat Vanguard are both described as living statues who judge or challenge those seeking audience with the Rat King. With such mighty challengers, it is no wonder that only the bravest and strongest will be able to overcome. For this, the Rat King will reward you by allowing accepting your service to his cause.
Your story echoes one who came before you. Not all humans shared the sentiments of those who had earned the Rat King’s ire, some may have wished to explore the realms below simply out of curiosity as opposed to greed. One such human came to the aid of the Rat King. The Rat King tells us that the Crest of the Rat was a ring forged by a human who was once among them. The ring was made by a sympathizer with the Rat King’s cause, for it draws to those wearing it any who would defile the Rat King’s burrow. The Rat King will also speak to you of the Lockstones, odd contraptions fashioned by human hand long ago, encouraging you to put these contraptions to employ against your foes. The Rat King’s burrows known as the Grave of the Saints and the Doors of Pharros are both filled with Pharros contraptions. Pharros the Vagabond himself was a legend who wandered the lands, creating contraptions to help those in sincere and dire need. So it would seem from all this evidence that the human who once dwelt among the rats and served the Rat King was Pharros the Vagabond himself.
It is never confirmed who the human chieftain was that the Rat King struck an accord with, but we can speculate. Laddersmith Gilligan will tell us that the Grave of Saints dates back to a time long before King Vendrick and Drangleic’s existence. We can also be confident that the Sunken Kingdom too, dates back to a time before Drangleic, as it is all but confirmed that Lindelt was established by survivors of the Sunken Kingdom and Lindelt were worthy rivals to Drangleic in their day (this being just one compelling reason). We can also see in the Grave of Saints, beings of a similar stature to the soldiers in the Sunken Kingdom. It may be that long ago the Sunken King was establishing himself on the surface, but heard rumor of an ancient dragon slumbering far below (interesting to note that the Rat King gives us a slumbering dragon ring). Driven by greed to see the dragon and worship his magnificence, the Sunken King led his warriors into the Underworld, breaking his word and driving the rats from their burrow homes. We know the eventual fate of the Sunken King, drowned beneath a sea of poison, perhaps this was justice for his belligerent crimes?
While the Rat King clearly detests those humans without honor, he does have a dream which hints that perhaps he only struck the accord long ago to protect his fellow rats, as opposed to out of distaste for the humans. His dream is that one day, rats and humans may yet live and die together, in peace.