Is this railroading?

I'm 15 and about to start my first full campaign I've been into D&D for about 3 years I've run several 1 on 1 adventures and spent what has to be at least 1000 hours of research into the game.

(this part is all unnecessary exposition into the campaign)I'm having a dilemma with the concept of my new campaign's first primary villain. He's a general in central city (the bard is a runaway noble from there) he enters the story as a lawful good hero trying to retrieve a crown, in all actuality he's just scouting the party potential for the kingdom. The party is to find out this truth at the end of session 1. From there on out he works as a sort of party benefactor and quest-giver.

(important part) At a certain point, the benefactor is meant to be forced to use a cursed artifact the party procured. By his will or another becoming possessed and thus the campaign's main villain. After being freed he slowly seeps into madness and tries to gain power to become the central city's new king and has to be killed this time around.

(The question) The purpose of the campaign is built around these events and I don't see such events being avoidable so is this still railroading if it's the main plot and villain?