Adironne's Guardian

Adironne's Guardian is a powerfully enchanted short sword that, rather than augmenting attacks, instead significantly enhances the defensive prowess of the wielder: it lowers the armor class of the wielder by 2, and provides a +2 bonus to saving throws. We may never know who Adironne was, but it can only be assumed that they would have met their demise much sooner without this sword than with it.

First encountered in G34: Charles le Ghoul.