He was infected with the Scarlet Rot, a sickness that transforms its victims into insane ghouls that have one desire that is all-encompassing: to Elden Ring Runes consume all they see. As such, Radahn now walks around battlefields littered with dead bodies on the back of a terribly small horse (whose name is actually Leonard) in the hope of the day when someone with the strength could end his misery.
Radahn's title in the Elden Ring is "Starscourge," and for good reason. In the past, a sorceress called Sellen claims that the huge warrior "challenged the spinning constellations" themselves. I'm still not sure what exactly Radahn was able to fight stars, but his victory over these celestial bodies was enough that they were frozen within the night sky. "He has the power that shatters all stars,"" Sellen adds. "If General Radahn were to die and the stars were to return, they would resume their movement." Simply said, the dude is as strong as a fuck.
In addition to his massive dimension, the strength of Radahn's is also due to his mastery of gravity itself, a school of magic that the player themselves could also benefit from with proper stats and knowledge. These skills not only make Radahn an imposing warrior, but it's how he's able to ride his horse with skin and bones without crushing the animal with his incredible weight. In fact, Leonard was the main motivation for Radahn to discover his gravitational power in the beginning, as indicated by this item's description: The Red Lion General wielded gravitational abilities he was taught in buy Elden Ring Runes Selia in his youth.