MoblinMan: You use the Revival potion on the Warrior (there's a small and big version, they differ in how much health and mana they restore on reviving the character). Also, the altars (yellow globes on the map) revive all your fallen party members and restore all the others' health and mana as well, just be careful not to use them when not necessary, as they can only be used once. The revival potions can be found as loot in the chests or after a battle, or purchased at the store (for which you have to find a teleport scroll first, which can be kind of hard).
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