He wrote everything he knew in books and scrolls, which he gave to his students. He’s a horrible educator, extremely strict and demanding of his students, so many youthful unicorns quit as quickly as they start. Having learned his ex-disciple Clover the Clever had founded Equestria and a learned new magic he began to truck to Equestria to see it himself. By the time he came to Equestria, he sees Celestia and Luna defeat discord and become alicorns. In where he nominates to tutor them and assist them in their newly assigned royal duties aswell as growing Equestria as a country.
In this he met many honorable heroes who collectively found the Pillars of Equestria, a honorable union of protecting the culture and safety from foes. But none of this could of been done without the persuasion from one ordinary pony named Stygian.
After Twilight Sparkle released the pillars of Equestria and consequently the Pony of Shadows, Star-Swirl was able to reconcile with Stygian and begin to learn more about the value of friendship.
Other Headcanons:
• Star Swirl’s sexuality was unclear, and a debated topic among scholars and fans alike. The only ones who knew where Celestia and Luna. • Star Swirl is a gay demisexual. Although he can be sexually attracted to stallions, he's never closely bonded to anyone with his intimidating personality. • Seeing the defeat of Discord from the two sisters who had become alicorns, Star Swirl had decided to tutor them to become rightful princesses to rule the ponies aswell as use them for studies on alicorns. • He grew fond of the younger sister Luna. And had even tried to accommodate for her different schedule. • Addicted to coffee and pulling all nighters. • He has anemia • He was extremely close to Clover the Clever and after his entrapment in Limbo, Clover continued Star-Swirl's research of the Everfree Forest and Tree of Harmony. • He loves to groom his hair, and is very fussy about being dirty.
MAJOR Diversions from Canon: • Star Swirl and the Pillars did NOT plant a seed that created the Tree of Harmony. They did, however, sacrifice themselves to stop Stygian. But this happened decades after Discord defeat. • The Tree of Harmony is a natural occurance with no outside pony magic involved • Celestia and Luna became alicorns because of the defeat of Discord as opposed to the flashback in season 4's premiere