Returning to the distinction between veteran noble raksha and Ishvara, let's look at them like comic book characters.
Noble raksha are your Wolverines, Iron-Men, and Batmen. Singular figures always retelling the same run of stories centered around the same personal themes, in worlds that narratively revolve around those themes.
Batman's story will always center around Gotham City. His strongest villains never really die, Arkham Asylum will never really rehabilitate or hold them, Gotham will never become less dangerous or corrupt... basically, the situation will never change in such a way that Batman becomes unnecessary or even needs to think of changing his methods. Batman never kills because "Batman never kills" is one of his Defining Principles. As a recent but enduring Major Principle, Batman hates working with equals because he's The Hard Man.
Darksied doesn't give a shit. Batman's story now revolves around him. When Darksied shows up, all of Batman's personal themes become unimportant, Gotham fades into the background, and Batman must either submit to Darksied and become one of his minions, a secondary character to Darksied's story, or Batman must join his story together with other raksha to form the shared narrative of The Justice League who opposes Darksied.
That's an Ishvara. A raksha so powerful, so charismatic, so narratively important that he makes the individual stories of other raksha revolve around him.
Ishvara are not your Lex Luthors, your Lokis, or your Magnetos. They are your Darksieds, your Thanoses, or your Apocalypses.
In "I'm A Marvel, And I'm A DC", Darksied made Joker into just another of his minions and forced the DC and Marvel heroes and villains to drop everything they were doing and focus on him.
In The Avengers, Thanos made Loki into just another of his minions, and forced all of the Marvel heroes, anti-heroes and villains to drop everything they were doing and focus on him.
Apocalypse is going to do the same thing in the X-Men: Age of Apocalypse movie. In his case, his horsemen are raksha nobles he bent into his minions, such as with Angel becoming Archangel.