Does not give +15% attack as you can see. It instead lowers your attack by 15%

that is not a bug. that negative atk means removing your mushiro magatama to fit the burai magatama in your max chakra.. and as you can see its color is darkened that means you cant wear it

neiby wrote:that is not a bug. that negative atk means removing your mushiro magatama to fit the burai magatama in your max chakra.. and as you can see its color is darkened that means you cant wear it

Wrong... As you can see he doesn't have the Mushiro Magatama selected, so it won't remove it's effect.

My question is how do you obtain it?

What neiby said was correct. Though the Mushiro Magamata wasn't selected, trying to fit a Magamata which goes over the limit of your chakra will automatically remove the highest + Attack Magamata(s) just to fit that particular Magamata.