For The King

Any time you're playing with multiple characters, there's a chance for this achievement. Eventually, one of your party members will die. As long as you have points remaining in your "Life Pool" (which is represented by hearts directly below the timeline at the top of the HUD) you will be able to revive the downed character. Simply move one of your surviving characters to the hex where the dead body is and press A to revive them.
If you're playing a multiplayer game, the person controlling the character being revived will also get this achievement.

When you’re in a battle with more than one ally present, when one has their HP go to zero, then your allies will have a new option in their turn “revive player”. Use this on the downed ally, they’ll come back to life with partial HP and the achievement will pop.