Starscourge Radahn was the child of Radagon - a red-haired champion of the Golden Order, the game's divine order and Rennala, queen of Caria, heading the Academy of Raya Lucaria, a school of sorcery. Therefore, he is the brother of the demigods Lunar Princess Ranni - later called Ranni the Witch - and Praetor Rykard. Radagon and Rennala's marriage cemented an alliance between the Golden Order and the Carian nobles, who had previously been deadlocked in battle. Since he was a young child, Radahn deeply admired Godfrey, Marika's First Elden Lord, and models himself after him, adopting Godfrey's lion motif.
While Radagon resented his red hair, a curse of the Giants and their Fell God, who had once posed the greatest threat to the Golden Order, Radahn viewed it as a positive, believing it indicative of heroic qualities. Despite following in Radagon's footsteps as a powerful general, Radahn also studied in Sellia, a town of sorcery located in Caelid. After realizing he was growing to a giant size and would be forced to abandon his beloved yet scrawny steed, he chose to study gravitational magic under the tutelage of a Alabaster Lord, a being originating from a meteor. This ultimately allowed him to lower his weight to almost nothing, and continue to ride his horse despite dwarfing the horse in size. Over the course of his life, Radahn is reputed as the mightiest demigod, an honorable warrior, an intelligent tactician, and a powerful sorcerer, gaining the respect and fear of many for his greatness. While known to enjoy battle, Radahn is noted to be kind, admired by many for his noble qualities, and is a caring leader to his personal army, the Redmanes. While studying under the Alabaster Lord, Radahn formed a friendly rivalry with Gaius, Messmer's friend and leader of his men. Gaius and Messmer acted as elder brothers to Radahn, with Gaius creating the "Blades of Stone" gravity sorcery as proof of their competition.
During his childhood, Radahn apparently made a vow with his half brother Miquella, who said that if Radahn upheld his vow, they would rule together as consorts in a "world of peace". Radahn used his gravity magic to "challenge the stars", succeeding in a "crushing victory" and halting their movement entirely. The Stars also halted the destiny of Miquella. This remains the case even when Radahn has long since lost his mind, and his death is required to restore their normal movement. Upon the advent of the Shattering, a war following the shattering of the Elden Ring, Radahn failed to capture the capital of Leyndell. Miquella's twin sister, Malenia, marched from the Mountaintops of the Giants and into Caelid to confront Radahn, seeking to kill him so Miquella could have his soul. The two of them fought to a draw, causing Malenia to resort to unleashing her curse of Scarlet Rot. Radahn was poisoned by Scarlet Rot so deeply that he lost his mind, and Caelid itself became a wasteland. Wanting to give their beloved leader an honorable death, Radahn's forces hold the "Radahn Festival" when stars have aligned, inviting warriors to slay their general and put him out of his misery.