Comment by Chives on Harry Potter and the Methods of Rationality discussion thread, February 2015, chapter 113 · 2015-02-28T20:28:19.056Z · LW · GW

A transfigured port key in his glasses does seem possible, or some kind of explosive device hidden in the transfigured ring.

EDIT: Fawkes is also a way out here, if Harry can delay an enemy attack for several seconds

Second edit (the serious one): The ring contains some kind of binding device. This could be the blinding potion Harry used in battles, a set of flash-bang grenades, or (most likely) a seventh-year variant of the blinding potion Harry bought off a student. This countermeasure will blind the unwarded death eaters which allows Harry to immediately fall to the ground and call Fawkes. Fawkes teleports Harry out and allows escape.