This 10th prestige (also known as a Capstone) only comes into effect once you have a Rune at it's 9th Prestige. Even without a rune being at it's 9th prestige, you are still able to buy a Capstone that you can use once you have a 9th prestige rune. You are able to design your Capstone Talent in it's entirely with this, both the passive effect and the boosts other Talents receive.