The First Descendant

This Achievement is story related and cannot be missed.

Each named region on the map is divided into smaller sectors, each with their own open world missions, called Normal or Battlefield missions. These can be done solo, or if another player starts the mission at the same time as you they can be added to your team so you can complete it together. Each sector has a set number of missions before it is completed, you can see this on the map screen.

Some sectors will have group instanced missions, called Duty of the Descendant, these will not occur in the open world map and will instead have either you or you and the other players you're matched with fighting your way through a unique map which will end with a named boss battle. And loot, lots of loot.

Once all sectors in a particular region are at 100%, the Achievement will unlock. And since you'll need to complete all the open world missions before being given access to the group missions at the end of the story arc, this Achievement is story related and will not be missed.