Diablo III Beach Towel.
The Diablo III beach towel was revealed on September 17, 2011, to be sold exclusively at Blizzcon 2011, to show attendees and virtual ticket subscribers.
It may be sold after the show as well, but that's not guaranteed, and many Blizzcon merch items are only available in limited quantities at the show, one year only.
The official production description:
"Made of 100 percent cotton and custom threaded for maximum comfort and absorbency, the 28"x56" Diablo towel is the ultimate bath or pool accessory."