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.

Pen Nib calculates all damage modifiers (Strength, Vulnerable, Weak) before the double damage, and is correctly shown in the tooltip before confirming your Attack.

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.