Templar Spellbane

Instant - Destroy this: Prevent the next 1 arcane damage that would be dealt to you this turn. If you've activated a weapon this turn, instead prevent the next 2.

Battleworn

Rulings

From the Compendium of Rathe release notes

When Templar Spellbane’s instant activated ability resolves, it generates a shielding-prevention effect that prevents the next 1 arcane damage (or 2 arcane damage total if you’ve activated a weapon this turn) that would be dealt to your hero. This can be over separate events of damage (i.e. 1 arcane damage, then 1 arcane damage).
To get the additional value for the shielding-prevention effect, you need to have activated a weapon this turn before Templar Spellbane’s activated ability resolves. If you do, prevent the next 2 arcane damage that would be dealt to you instead of just 1.

Keyword notes

Battleworn - Equipment that wear down after each time they are used to defend. If you defend with a card with Battleworn, put a -1{d} counter on it when the combat chain closes.

When you defend with a card it remains on the combat chain until the chain closes, therefore you cannot defend with an equipment multiple times on the same combat chain.
A -1 counter stays on the card until the card is destroyed or the game ends.
If the defense of a card that is defending is below zero, its defense value is considered to be zero.
When this card is destroyed, it is sent to your graveyard. (Not when it has 0)
When this card is destroyed, it is sent to your graveyard.
If the defense of a card that is defending is below zero, calculate as if its defense is zero.