The purecrystal/twisted sword will be useless in chapter 5+. Some tragedy will happen that sets the plot in stone, like kris leaving or susie going full villain.seam will laugh at us for our naive and wasted hope
BUT
The purecrystal is retroactive. It's dropped by jevil with new dialogue.
You can craft the twisted sword at the end of chapter 2. Equipping it completely changes chapter 3.

Twisted kris fucks with the plan and has the knight arrive early. Tons of shit changes. It lets you KILL the knight.

This is how you unlock chapter 6