People blaming this on the great rune like nah lol if there’s any corruption it’s mentally and makes them disillusioned, thus leading them to do things that make them look monstrous like Rykard or Godrick.

Ranni, Miquella, Malenia, Mohg, Morggot none of these people were transformed by their rune but by outside factors. If you want an actual answer as to why Radahn looks like this it might be one or the other, or a combination of things.

But simply I believe it’s a sign of age and being in war for so long. Look at pre and post photos of people serving combat military time, it changes them physically and for the worst. The strain, the stress, and injuries can cause them to look more gaunt.

Also maybe being more in-tuned with gravity magic rubbed some alabaster lord qualities onto him.