This is awarded for defeating the Boss Cerberus during Chapter 14 - Homecoming.
He was only Level 45 and since I've already beat the game and I'm level 87 he was a simple fight.
Assuming you already played and finished the game you'll need to chapter select to get back to this point once you've installed the Royal Pack.
Back up your save to another slot before you chapter select as this will reset a lot of progress. Chapter Select is basically a NG+ where you pick where you want to start.
During the Main Quest Line The Cure for Insomnia you'll encounter this boss on your main path.
If you just picked up Royal Edition and you're playing the game normally for the first time, this is just something you encounter as the story content has been increased with the new Royal Pack.