Darkside

Darkside is one of the four heroes of Chaos Immemorial, alongside his sister Sharp Mind, Trail Blazer and Bare Grills.

History
Darkside and Sharp Mind were members of an upper class, noble family in Canterlot. From a young age, Darkside expressed an interest in magic and, as soon as he could, enrolled in Princess Celestia's School for Gifted Unicorns. He received his cutie mark when, to pass the entrance exam, he summoned a bolt of lightning from a cloud outside the room (previously, it had been thought impossible for unicorn magic to overlap with pegasus magic).

After the events of the 1000th Summer Sun Celebration and the defeat of Nightmare Moon, Princess Luna returned to Canterlot and decided to pick a personal student, much as Princess Celestia had Twilight Sparkle as her student. Of the unicorns in the School for Gifted Unicorns, Darkside was one of the most powerful and, as such, was chosen to be Luna's student (much to the disappointment of Sharp Mind who had hoped for that honour for herself).

Darkside was sent by Luna to visit Twilight Sparkle's library in Ponyville, and procure some books which Luna wanted for research that Canterlot's library was missing. Upon arrival, he was a witness to Sight Seer being attacked by a group of Cultists of Discord. Luna then sent him to live in the Ponyville Observatory, so he could protect the town against the Cult of Discord and perform further research into magic.