BloodStone
Since Katsou is basically an alchemist like those in wuxia novels , his domain expansion could be something like a cauldron to refine whoever is in there. For example he can use Positive cursed energy to purify and enhance his allies treating their wounds or even buffing them. Secondly for his enemies well , they become ingredients for making his like Premium pills. Those are some just ideas and for a name it could be called something like the Heavenly cauldron