Altar of Resurrection
Altar of Resurrection is a rare item used to revive a unit that has died during combat. You will not collect many of these items during normal play. You begin the game with 2 and you can find 1 more on the first mission (Tenne Plains). After that, you only find a few more hidden during missions and you can sometimes win them by defeating enemy units or by killing wild Sphinx.
If you are having trouble reviving units, make sure you visit the Witch’s Huts instead. This costs Goth, but it can be done as many times as you can afford it.