This setting takes place with medieval level technolpgy. Upon death, all souls leave the body and go directly to God to become one with its consciousness. However, there are other competing gods who wish to claim human souls for their own purposes. Souls provide gods with power and make them stronger. These gods have their own disciples and cults who work for them by making human sacrifices in exchange for dark magic.
All gods have their own requirements for a sacrifice. IE: a full moon, certain ingredients, a certain location or season, etc. But there is one requirement found across all methods that is necessary for this ritual to work. Ancient runes and sigils of various sizes that represent the god in question must first be placed into the victim to make the sacrifice specific to that deity. These runes must be carved deep into the skin, past the epidermis and into the hypodermic layer itself. These runes must be carved from head to toe, back to front in a specific and intricate pattern. Then the person is killed in a particular way to complete the sacrifice.
This ritual is long, painstaking work. The individual could die from blood loss, infection, or simply shock. The person needs to be kept alive through the ritual until the end. How best to achieve this?