Mound-makers is a covenant that can be gained in the Undead Settlement by speaking with Holy Knight Hodrick before the Curse-rotted Greatwood.
Reaching Hodrick alive can only be done by approaching the Hollow Manservant with a cage on his back, on the south end of the town before the Cliff Underside bonfire. If you fail to do this you’ll have to work on Sirris of the Sunless Realms quest line to get this much later in the game.
Mad spirits, Mound-makers can be summoned with a White Sign Soapstone or Red Sign Soapstone, though being summoned with either does not prevent the crazed spirits from turning on the Host of Embers or other phantoms.
Defeating the host, phantoms, or players defeating Mound-makers will result in a Vertebra Shackle , which can be turned into the shrine at the Pit of Hollows in Undead Settlement.