Dragon's Dogma 2

Spoilers.

When you get to the end of the game you'll be faced with a choice to walk away from the Dragon or to attack it, choose to attack the dragon.

You'll then be riding the dragon, simply don't do anything, just stay on its back until you reach your destination.

When you arrive sit on the throne and you'll unlock the achievement once the credits roll.

Don't worry though, If you want to head to the Unmoored world to get those achievements simply load your last save once you get past the credits which will take you back to the throne room, then speak to the blue figure twice. When on the dragon again head towards it's heart and use the Godsbane blade on yourself.