From Dark and Darker Wiki
Soul Heart
To resurrect an ally you will need to locate their Soul Heart which is located in their chest armor slot. To acquire their heart you will need to remove their chest armor piece. Equip their soul heart to you utility bar then proceed to look for an Altar of Sacrifice. After locating a Altar of Sacrifice, hold your team mates soul heart in your hand and press interact button. This initiate a channel action that will resurrect your team mate when the bar is full. Being hit or moved will cancel this action. You can't take enemy's soul hearth.