So I picked this item up from the store around Chapter 2, has a IV-type attack called 'Mortal Regret' that has a whopping 67 total attack power (meaning it hits for 1 damage against the early-game enemies), along with potentially adding Poor Aim.

Should I be bothering with this weapon at all? It's a pain to increase the Mastery of it, but I'm sticking with it out of hope that it'll become usable at some point.

EDIT: It seems at a mastery of 13 it unlocks Root Blade. Still uselessly weak compared to the Iron Sword +3 you begin the game with.