Aeterna
Piece the Marks of the Ruins together on the Aeterna Map to rebuild the City of Aeterna.
Kill monsters, open all Candlelights of Sacrifice to summon the boss, and then kill the boss.
Kins of Aeterna will appear when "Kins of Aeterna are wandering around in this stage." Killing Kins of Aeterna has a chance to grant Aeterna Reverberations, which can be given to Kandel to open the Ruins of Aeterna. Completing the challenge of the Ruins of Aeterna to get Marks of the Ruins.
Piece the Marks of the Ruins together on the Aeterna Map to cover the reward points on the map. Continue until the Aeterna Map is filled up by the Marks of the Ruins to rebuild the City of Aeterna. You can also fill the Aeterna Map with Aeterna Tiles to make it more complete.
After the City of Aeterna has been rebuilt, you can enter the city and kill monsters to obtain keys, which can be used to open Candlelights of Sacrifice for rewards. After opening all Candlelights, you will be able to summon the Aeterna boss. Kill the boss to obtain the final reward.