In the KJV:
“How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!”
In the NIV:
“How you have fallen from heaven, morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations!”
The NIV does not give a clear indication of who the “morning star, son of the dawn” actually is, and this becomes very confusing when attempting to understand the nature of this passage.
The KJV makes it very clear that the “son of the morning” is Lucifer, the fallen angel who is the enemy of humanity. He is mentioned by name in Isaiah in the King James Bible. In other instances he is called Satan (meaning adversary).