When you have collected enough Mark of the Ruins, socket them onto the Aeterna Map. Once the Aeterna Map is complete, you can enter The City of Aeterna through the Monument of Aeterna or Kandel. Defeating monsters in The City of Aeterna will grant you Energy, and collecting a certain amount of Energy will reward you with Kindling. Offering Kindling to the Sacrificial Bonfire will grant you generous rewards. When the Sacrificial Bonfire progress is full, you can challenge the boss of The City of Aeterna. The more Bonfires you have, the better the boss rewards will be.
Torchlight Infinite Flame Elementium