[the REAL Minos Prime]

Shine Sword is mostly a gimmick weapon. Extra damage and a transformation. I more often see people take Shine Sword just for the transformation, not intending to actually use it as a weapon.

To encourage its use as a weapon, and cement its role as a paladin's weapon, I propose a simple buff - 


Shine Sword can be used to cast Mercalan domain spells.


If this is considered to be too overpowered, then I'd add the caveat that the user must also be under the effects of Pray in order to cast with it, to further lean into the Paladin style of a religious knight whilst also not slinging Shine Rays and Malmelos left and right. However, I don't see why it being able to cast Mercalan at all times would be op, given that it's both an 8-Star and a black book weapon. A weapon that requires either a really lucky drop or a special recipe should have some special quirks that make it stand out.