Defensive Boost is a apprentice-level Alteration spell added by the fourth plug-in Spell Tomes.
As with all spell tomes, Defensive Boost spawns randomly depending on multiple factors such as the Hero's level and spell level.
However, for the best odds of finding a Spell Tome, search boss-level chests in Necromancer and Conjurer lairs.
The effects of the spell are:
The Magicka cost of the spell is 57 points.