This is the Kingdom Hearts / King Mickey Mouse Power Costume Power Disc for the Disney Infinity 3.0 edition. It's a Costume Change Disc which, when placed under Mickey Mouse on the Disney Infinity Base, transforms him into King Mickey from the Kingdom Hearts game series.

I picked up this disc in person at the Disney Interactive panel at the 2015 D23 Expo in Anaheim - and it's been in storage ever since. Only 5,000 were ever made - and it was never sold in retail. 

The item is mint and unopened - see photos.