Argyle Starshine was an avid collector of relics from eras long gone, with the hope of proving that ponies used to be united. When Sunny succeeded in reuniting the three breeds, bringing magic back into the world, she unwittingly released an ancient foe of Ponykind.
Spoilers for the Gen 5 movie.
Gen 5 Bingo Contest - Sprout, Sweets, Grogar, Everfree Forest, Dragons