Double Dragon

Double Dragon is a dungeon & raids achievement earned for killing 6 Twilight Fiends in the Twilight Realm and then defeat Valiona and Theralion in the Bastion of Twilight.

Strategy
The fight consists of two phases, with one dragon airborne and the other grounded during each. When they are about to switch phases, the airborne dragon will use an ability (Deep Breath for Valiona; Dazzling Destruction, a void zone ability, for Theralion) that transports those affected to the Twilight Realm. The Twilight Fiends can be found here.

Trivia
This achievement is likely a reference to the Double Dragon video game series.