Relic fusion is a mechanic in the Devil Children series.
Relic fusion is a special fusion where one takes fossils that can be bought, found or obtained from battle in the game and fuse three of them together. This can result in either a zombie version of certain demons or a regular demon being born. No matter the order the demon will be the same.
Resurrecting Zombie fusion will result in a zombie demon being brought back to life after its soul is fused with it. Souls, especially the high level ones, are typically hidden or obtained from demons that can only be fought after beating the game.
Relic fusion is overseen by Angrboda in each game.