Instructions
Build a Temple
Click on the Capital in the over-world map. You cannot do this during combat.
Select the "Construction" button and then "Temple." This builds a temple in the Capital, giving the player the resurrection option out of combat.
Open the garrison window and select the unit you want to resurrect. Push the "Resurrection" button in the lower, left corner of the screen. Again, this only works outside of combat.
Revive
Level up a spell-casting unit. You gain experience by fighting and winning battles. Take the unit through several successful campaigns to gain its level.
Select the "Revive" skill during level up. This allows that unit to cast a spell during battle that revives fallen party members.
Open the spell menu and cast "Revive" during combat. The fallen warrior will return to life, but only with partial health.