I think maybe we are overthinking the quote "unlocking the relic's true powers" here, my thought is that the relic itself is at its peak in zenith form and its "true power" is to be used to create a completely new weapon not relic anymore but something greater that can be built upon as the game progresses. This could very well be Zodiak or what ever crazy term used to descried the weapon but seeing as relic's are now zenith I can't see the new form being considered a relic anymore (more like the new weapon can only be acquired or created from a relic weapon in is "PEAK" or Zenith form).

This would make more sense to me!