VW cv joint bolt. This is a set of 6 bolts for the CV joint found on many Passats and Audis that use the tripodic style inner joint. They measure 10mm x 20m and use an 8mm triple square bit to remove. Sold as a set of 6.
If you're removing your driveshaft, you'll need an 80mm 12 point Allen star tool for the inner bolts, and the 30mm axle nut socket for the outer bolt. New axle nuts can be ordered as part #19331.