Courtesy of ringsidenews.com:
Both Sting and Undertaker are scheduled to appear at WWE SummerSlam next month.
Dave Metlzer mentioned on the latest installment of Wrestling Observer Radio that WWE is looking to throw a gigantic SummerSlam weekend. Sting was scheduled to return at some point, but his plans for WrestleMania 32 are not known except for the fact that he will not be wrestling The Undertaker.
The two are not expected to make surprises appearances, and WWE will announce their returns well in advanced.