First regeneration on Gallifrey

The first regeneration on Gallifrey was an important moment in Gallifreyan history.

According to redacted parts of the Matrix, the Timeless Child's regeneration after falling from a cliff was the first regeneration on Gallifrey. The Child brought regeneration from its point of origin beyond Gallifrey. (TV: The Timeless Children)

The Scrolls of Gallifrey indicated that Rassilon experienced the first regeneration on Gallifrey. Following the Eye of Harmony's sealing, exposure to anti-matter weakened Rassilon's first incarnation. When Rassilon was confronted by Tussan and the Chancellery Guard, the stress caused Rassilon's miraculous regeneration before their eyes, and Rassilon became the first of the Time Lords. (PROSE: The Legacy of Gallifrey)

By these accounts, the momentousness of the first regeneration led to further research into the phenomenon which brought the establishment of Gallifreyan regeneration. (PROSE: The Legacy of Gallifrey, TV: The Timeless Children)