During the mod system transition between Update 6 and Update 7, all of the previous mods were turned into Fusion Cores to be used in the Mods 2.0 system. During this conversion, all of a player's preexisting mods were turned into a random core between the rarity of Common 0/10 up to Rare 10/10. After the transition, Common Cores were maxed to 3/3, while Rare and Uncommon topped off at 5/5. Hotfix 14.0.9 renamed these cores as "Ancient Fusion Cores" and made available for trade through the Dojo trading post.
Ancient fusion cores can currently only be obtained through trading with other players.