You can access the achievement diary from anywhere with the command
You'll receive different rewards based on the tier of achievement you complete!
Each achievement will give you a gilded parchment, ranging from 1-4 depending on the tier.
Completing all tasks in a tier of the achievement diary will reward a piece of the draconic armor.
Draconic armor is the best-in-slot regular armor (excluding crate pieces) and provides a huge set bonus when every piece of armor is equipped.
The Draconic Armor Set