Every 10th Attack you play deals double damage.
Pen Nib gains 1 charge every time an Attack card is played. The charges carry over between fights.
At 9 charges, Pen Nib will grant double damage on the next attack, and then reset to 0.
The bonus applies to the entire attack card (so multi-hit cards will get it on every attack.)However, note that Double Tap produces a duplicate card, so in that case the bonus will only apply to one of them. On the other hand, a double-tapped card counts as two plays for the purpose of charging the nib.
Pen Nib is a Common Relic.