TD bonus is a rare item property that adds to target defense. It can be found on armor and shields in the treasure system, and also added to shields and armor (or to existing bonuses) at high-end pay events such as Cessation of Coraesine Field (CCF) and Duskruin Arena (in the Scrip Shop).
A bard loresinging to an item with a TD bonus will have the following message:
The harmonics generated tell you that the armor serves to protect from magical attacks.
To give an example of the rarity and cost of this service, back at Return to Coraesine Field an 8x medium shield and a 7x fusion shield were estimated at 5 million silvers (after a $199.99 event ticket fee). In the August 2016 Scrip Shop, adding +10 TD to an existing TD bonus item started at 250,000 bloodscrip, and ended at 312,500 bloodscrip (prices increased per sale).