Who is Atriox?

I’ve not read or looked at any of the Halo books or comics and what-not, I’ve only played the Halo games. Can anyone confirm whether or not this Banished faction have ever been mentioned before? Who is this new character Atriox? Why are there elites sided with this new faction? I need answers! I am very hyped but I need answers!

far as I know this is all brand new to us