The Arrival is the moment the Relentless is officially commissioned for the 47th. This acts as a big chapter in our collective story. Having said that, this is the culmination of countless hours of previous story coming together here. For the truest experience consider reading as much of the related stories in the Galactic Holonet Archives. This includes but is not limited to.
The Imperial Army Chronicles:
-The Hand of Fate -An Echo Through Time
These are the stories that most set-up and are referenced in The Arrival.