This is my personal take on the lore and it tries to be as accurate as the card-art as possible but it is not official
With Gem-Knight Seraphinite returned to the Extra Deck, you have a chance to activate your second Brilliant Fusion to Fusion Summon again
So, for example, if your top two cards are Brainstorm and the transforming double-faced card Delver of Secrets // Insectile Aberration, you will end up with a modal double-faced card in one of the following permutations: Brainstorm // Delver of Secrets, Brainstorm // Insectile Aberration, Delver of Secrets // Brainstorm, Insectile Aberration // Brainstorm
1 fantastic oversize card featuring Mewtwo-EX(suitable for display)
Most impressively, Dragon Shields website also offers the option to craft sleeves, featuring your own art or photos