Welcome to Biquestria

Starswirl recalls one of the first legends he has to rescue: A bicorn. Bicorns are Unicorns descended from ponies that do not have the purity they do, but can develop it by their trademark difference: two horns, which harness positive (light) and negative (black) magic, as when combined, make them excellent Equinox masters. A distress signal has been unleashed to Starswirl from his bicorn student Galaxy Rush, which indicates he is in danger. Thus the heroes must travel to Biquestria, the land of bicorns. They meet their leaders, Prince Major Pulse and Prince Minor Pulse, who have a similar but short history to Celestia and Luna, as they explain that Galaxy had went missing as the possibility of a returning foe has concerned him. This threat comes to their doorstep in the form of a Tirek-like Dragotaur (Dragon Centaur) named Apostopheles, who refuses to fight against cheap tactics since a war against him and the Bicorns was solved that way, and demands the war be reinstated and fought fairly, originally through an anti-paradox relic that alters anything it finds aberrant to a certain standard until Galaxy debunked that and banished him. Now he's back with something stronger that makes him somewhat unstoppable, as he is invincible, invulnerable, has Herculean strength, a healing factor, and is also immortal, through a relic infused in his chest that, like the Alicorn Amulet, corrupts the user in exchange for it's power, and with it proving to be hard to remove, and Apostopheles seeking the anti-paradox magic and it's still-living creator Santa Maria, the heroes must find Galaxy and get him to confront this threat before he gets what he wants.