Spellbook needed to learn 'Fortitude'.

Required:
Paladin - Lv. 70
Dark Avenger - Lv. 70
Temple Knight - Lv. 70
Shillien Knight - Lv. 70
Destroyer - Lv. 70
Tyrant - Lv. 70

To obtain this item, hunt the Conjurer Bat found in Blazing Swamp.