Final Fantasy XV: 10 Perfect Fan Theories That Make Everything Better
3. Ravus Changed After Kingsglaive
In the FFXV prequel movie, Ravus tries to wear the Ring of Lucii,
only for his arm to be set aflame (why he wears an artificial
replacement in the game). Before donning the Ring, he has two blue
eyes – yet in the game we see he has heterochromia – one blue
eye, and one purple.
It's worth noting that purple was the colour Noctis' eyes were supposed to turn back in FF versus XIII when using his powers and is the colour they turn when summoning Astrals in battle.
This theory ties in with the earlier one that Noctis is part-Astral. Ravus serves as a reminder – on more than one occasion – of what the power of the crystal would do to a normal being.
When he tries to wear the Ring, unlike Noctis, his genetics aren't able to handle the power and he is forever scarred by it. And when he encounters the Starscourge, unlike Ardyn and Lunafreya who absorb it, he is overcome by it and transformed into a daemon.