Different reactions were heard with regards to Final Fantasy XV's ending. Most were disappointed, others said it's just "Okay", while some are asking "What really happened?" This article will help those curious brains figure out what really happened to Noctis and to his adventure being the heir of the throne and banisher of demons. We'll also run through some small details that you might have missed while playing the actual game.
Hundreds of years ago, Ardyn Izunia was given powers by the gods to save the mankind, Ardyn absorbed the Starscouge as well the demons that time thus giving him unwanted immortality, that gave the nickname "Saviour of Lucii" this was proven by the book of "Cosmogony" the cover of the books was Ardyn and Shiva (lately revealed to be Gentiana) the book cover illustrate the exact moment when the Astrals gave Ardyn the powers that he can use to save mankind.
The First King without the crystal's power/blessing and Ardyn being loved by the people was corrupted by jealousy, he betrayed Ardyn who also was eventually left out by the Astrals because he became corrupted after healing too many people and the demons that he absorbs manipulates his mind.
Izunia was the name of the man who betrayed Ardyn, Izunia is also where Noctis's bloodline came from. (But Noctis's last name was Lucis Caelum right?) seems to be that the man who betrayed Ardyn which might be Noctis's ancestor stole the name "Lucis Caelum" which is now used by his successors and as well as Noctis BUT Noctis being chosen by the crystal proves that Noctis is really a Lucis Caelum. It can be concluded that Ardyn is Noctis's great great grandfather.
While some theories say that Ardyn was betrayed not by the First King but by his own blood, maybe brother, son etc, thus making him wanting to wipe out his own bloodline Including Noctis. One of the supporting facts is when Ardyn told Noctis his true name not Ardyn Izunia but Ardyn Lucis Caelum.
Ardyn has all the starscourge inside him as well as the demons use his powers which were shown during the movie KINGSGLAIVE. Ardyn wants the ring's power to be complete thus making Noctis the king, That is the only time where Ardyn can fulfill his goal of vengeance, Ifrit one of the Astrals was the one who created the starscouge and wants the demons to be free protects Ardyn, because Ardyn has all the Starscourge inside him. If Ardyn dies, the Starscouge will be lost with him.
Ardyn's plan was to let Noctis harvest all the power of light and all of his ancestors making him the king. Having all these powers, Noctis can banish the Starscourge once and for all, but the darkness became too powerful that a sacrifice for the chosen was needed to remove all darkness.