You use your Mirian currency in the upgrades menu. You get Miran by completing challenges, finding artifacts, finiding Ithildin along with doing mission and legends. You play the game long enough this will just happen.
To get there.
1. Press
2 Use
to scroll down to upgrades
3. Hit
to get to your attributes page
Once there scroll over to the far right and you will see your dagger rune unlocks. It will let you know how much Mirian you need to unlock each tier.
Simply play the game and when you have the required Mirian unlock each upgrade. Once you have all 4 upgrades in that tree bam achievement unlocked.
Pretty much any task minus the story missions net Mirian. By hitting
once more it will take you to the progression page which will show you what rewards you will get and also how far along in the game you are.
Hope this helps!