When you say "it doesn't make sense", its just done to eliminate answers you dont like. What about the game has said those not of royal lineage must die when using the ring? It just says a blood price (sacrifice) must be made. Hell, even the rightful bearers pay a price. They start to develop scars after use.

It doesn't make sense means it's not consistent how it is portrayed all along, not that it eliminates answers I don't like - because there are no answers. Alternate Verse makes a pretty poor impression of the Ring of Lucii considering Ignis actually suffers no long-term injury. But that's fine, since it's a what if.

I would have preferred a proper explanation of it, not the fact that the Kings can demand whatever when they feel like. "Rightful bearers" include Nyx, Noctis and Ignis, because they can bear the ring without automatically dying in the process (non-rightful bearers are outright killed by the Kings), so it's redundant that you mention that. When a Glaive as Nyx is asked for a life, and Noctis has to surrender his life to enter the Beyond and employ the power of the Kings (which also consumes him), Ignis is deemed worthy of using the ring only at the cost of his sight, and this has virtually no explanation whatsoever (heck, he can even see traces of light as stated elsewhere in the main game, and the only scar that appears is just over his left eye). Both Nyx and Ignis find themselves having to resort to the ring to safeguard the future (Nyx protecting Lunafreya, Ignis protecting the Chosen King), yet Nyx is the one that gets royally screwed the most because...because the Kings felt bitchy that day or something.

Your right, its not consistent. That is why I am asking where did the did the idea of death being the blood price come from. Nyx and Ignis did not gain access to ring because they were chosen. There are people who use then will die, then there are ones like Nyx and Ignis, while not lineage have potential to use it but at a offering, due to their connect with family. Remember, both Ignis and Nyx have limited access to the lineage magic and both are directly responsible for aiding the lineage. Makes sense why the Kings would be willing to allow them use. The access was granted because what they were willing to sacrifice for it was consider worthy. Noctis was not killed by the ring. He in a sense had to commit suicide to allow the ring to to his soul to the other dimension.

I'm not sure if it was ever stated in the game itself, though the FF wikia does remark that it asks for a blood price.

http://finalfantasy.wikia.com/wiki/Ring_of_the_Lucii

Blood price does not mean death. Chopping off your arm to have your little bro spared can be considered a blood price. We clearly see that in FF15 world as well of it not always resulting in death. So why think it means death?  Ignore the wiki and look at the game you played.

Eyesight can't be considered blood price, not through normal means unless Kings love rethoric.

If anything, the fact that Ignis can come out unscathed as much as he does after using the power of the ring to fend of none other than Ardyn cheapens the idea of the sacrifice itself. Which is why in context, it's still weird that Nyx was asked for a life but Ignis was merely asked for his eyesight.

Nope.

Nyx is a soldier. He lives only to protect the King and its people.  Could he have offered something like an arm? Sure, but then that would leave him unable to serve, If he cannot do what he has lived is life for up till now, then what value is left in his life. To him that is his ultimate sacrifice .  Ignis sacrifices his sight, but with that his passion as well. He can't cook anymore.  In our own world there are people who lose the ability to do what they love and they go into a deep depression and even suicide. You are trying to scale sacrifice and it's no good. 

Well in terms of Ignis why he only lost his sight compared to Nyx loosing his life we can see it has something to do with how much power they accessed from the ring. When Ignis looses his sight in the regular path he is able to fight Ardyn but he obviously was still not really a match for him so he only barely accessed the rings full power, in the alternate path for Ignis to use more of the rings power he has to sacrifice his life and we see his entire body is getting destroyed, and he managed to temporarily defeat Ardyn, he only survived cause Noctis asked the crystal to save him. As for Nyx when he used the ring he went ape shit and practically used Knights of the Round against diamond weapon and was teleporting and fighting in the air all DBZ style so he died since he used so much of the rings power. Plus as we could see the ring is kinda picky, in Kings glaive we saw Ravus only lost his arm, while that other dude got completely fucked after putting the ring. So in conclusion the kings are just dicks.

As for Ardyn trying to kill noctis in episode ignis it was obviously a ploy, Ardyn is known to be a scummy troll of man who likes to mess with people, so he only did that to be a dick and make everyone freak out. He wasn't going to actually kill Noctis since he needed him to become the true king. Plus if you look at the mural in near the throne room you see 4 dudes who seem to represent the bros, its obviously supposed to be a prophecy type thing, one of the 4 dudes in the mural is blind, so Ardyn was most likely just trying to force things to be just like the prophecy.

He's morphed into her, Ardyn likes to play games. Just watch some YouTube videos on it. There's tons of evidence to support. The final scene alone nails it. Why would Arenaea just stay there and let Prompto drive off? Then seconds later Ardyn is just standing where Arenaea was lol. Not only that but Arenaea lies to Prompto, why would she lie to him? Arenaea and Ardyn are conveniently never in the same scene ever during the episode. She wears her clothes exactly like him, look at her scarf for example. Ardyn taking down the factory means little to him, if you play chapter 13-2 you will see he has little care for the Empire really, and it's all just a game of vengeance for him. He wants to get in your head. There's other stuff that I've missed but it's been a while since i played that part.