This is my secret weapon achivement.

[Fatelighter]

Hi everyone, this is the one achievement I'm not seeing much information on, so I thought I'd drop this here in case it helps someone in the future.

The requirement for this achievement is to "Fully upgrade the Energy Core"

To do this, you need to solve shrine puzzles (or, at least one) and then interact with the shrine again after it's been activated.
This allows you to spend Energy Prisms to unlock upgrades. It's as simple as maxing this out.

I had over 45 hours in the game and beat the final boss before I even knew this was a mechanic.
Hope this helps someone!

[Sou Kurosawa]

45 hours and beat final boss?
You kinda just rushed the main story bruh,
i'm at 50 hours and i'm at region 3