The mad may find malformed vertebrae with a queer symbol on its inside, proof of the shackles of the Gods. They then pledge themselves to the Mound-makers covenant. The mound-makers wish only to add to their mounds, becoming mad spirits. They are blithe to those around them, for in their minds, any kill might lead to another shackle.
Members of the covenant are tasked to collect special bones discovered in the corpses of their victims. Only one such bone is found in the vertebrae, and the Mound-makers believe it to be a shackle of the gods. In their minds, each victim is another connection, an addition to the family.
You can speak to a certain Cage Spider for a hint of how to join the covenant. The shrine of the Mound-Makers can be found in the Pit of Hollows, beneath the Rotted-greatwood boss in the Undead Settlement.