If I understand correctly, this item is incredibly overpowered, allowing you to inflict a 5d6 bonus or a -15 penalty to a d20 roll up to twice a day for a mere 7,200 gp.

Compare this with, for example, the Fatespinner. The capstone ability (character level 13th) allows a single opponent's save per day to suffer a -10 penalty. And it is a very powerful ability even at that level.

Perhaps if this inflicted either a +2 or -2 bonus, usable 5 times per day (non-stacking) it would be balanced for the cost. As it is, it's so overpowering it's not even amusing.