In Dark Souls, the player must obtain four Lord Souls before entering the Kiln of the First Flame. These souls come from Seath the Scaleless, Gravelord Nito, the Bed of Chaos, and the Four Kings.
The requirement represents the game’s final major progression gate. After placing the Lordvessel, the player can pursue the four Lord Soul owners in a largely flexible order. Each encounter leads through a different region and a distinct part of the world’s history, from Seath’s archives to the ruined domains of Nito and the Witch of Izalith.
The Four Kings are sometimes overlooked because their battle occurs in New Londo Ruins and is tied to a key obtained from Ingward or the area’s progression. The player does not need to defeat every major boss in the game, nor every optional boss, to open the kiln. The four Lord Soul powers are the specific requirement.