Piercing is a star within the Constellation The Ritual in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It increases the amount of Armor your physical attacks ignore by [x]. You can put 100 Champion Points into this passive ability for a maximum of 5280.
With each point invested, the increase in the amount of Armor your physical attacks ignore diminishes. That is to say, for the first 10 CP invested, you would gain a 1003 increase in Armor Penetration. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 897 more Armor Penetration. This is 106 less than the increase gained in the first 10 CP.
As such, spending just 37 CP into Piercing will already gain you a 3184 increase in Armor Penetration of your physical attacks. This means you will have gained 60% of the Maximum increase possible (which is 5280) by spending just 37 out of the total of 100 CP. Therefore, beyond this point, the increase in Armor Penetration received per 1 CP used is very small and not as worthwhile.