Shadow Crystal in Chapter 2
So, you defeated the secret boss of Chapter 2 and got his shadow crystal, then you went back to the castle and the shopkeeper told me you could get the one from Jevil’s if you defeated him.
How does that work?
You can go back and simply do the encounter.
Once you have the item the game will know and upon checking that little hole in the wall on the left most part of the castle town the item will be inside it.
Haven’t confirmed it, but if you play Chapter 1, defeating Jevil, and then save, when you go back to chapter 2, you may find something to the right of the Castle Town entrance. Seam says something about being able to recover secret boss items if you’ve ever previously fought them in prior chapters.