For the past month and a half, I've been DMing a 5th Edition campaign set in Primeval Thule. One of the major differences from other settings out there is the emphasis on a pre-medieval, ancient feel in keeping with its pulp sword and sorcery inspirations. There were no churches, there were woodland groves and ziggurat temples. There were no plate-clad knights, instead there were barbarians and gladiators. Although there are still some standard fantasy trappings as part of the D&D ethos, Primeval Thule does a pretty good job of emulating a realm different than the Tolkienesque model.
We're at a point in the game where the PCs have enough loot and resources to begin establishing their own stronghold, complete with hired servants. I set about making a stronghold-building-in-progress set of house rules to provide some in-game boons for certain purchases. One of them was a sacrificial altar dedicated to the gods, upon which the PCs can give up some of their loot to be consumed in exchange for temporary benefits. The idea was a huge burning brazier whose fires could burn down even metal (because a Cleric Did It), but I left things to the imagination for when the players decided to obtain it.
Then I noticed something. A distinct lack of rules for sacrifices. Going even further, I could not find other examples in other Dungeons & Dragons books beyond a generic role-playing trope or the exclusive providence of evil deities.
A common cultural and religious practice in many real-world cultures is that of the sacrifice, or a material offering to the gods and spirits. Although the reasons and forms it took varied, the general intent was giving up a material possession in exchange for divine favor. While human sacrifice is generally the most iconic, animal sacrifice, the burning of a portion of harvest, and the casting of gold and jewelry into watery depths follow a similar intent of giving up something to entities beyond the mortal realm.
While it is reasonable that a devout PC can dedicate monster-slaying as honoring his god (a 'sacrifice' of a sort) or burn down a rival's house to placate vengeance spirits, the intent of these rules are sacrifice of a more formalized affair. Basically an elaborate ritual in a location, where a supplicant gives up something belonging to themselves in exchange for a boon.
*I may type up rules for Pathfinder and Swords & Wizardry/Basic D&D in due time and if there's enough interest in the subject matter.
Sacrifices are conducted at a shrine, a place meant to honor a god (or gods). The shrine can come in many shapes and forms, but must be worth at least 1,500 gold pieces and cannot be portable or mobile. The shrine must be regularly maintained by a person proficient in Intelligence (Religion) or belong to a magical class whose spells come from the patron deity in question. Different deities might have different boons, but here are a few of the more common ones:
Burden-Bearer: Transfer an equivalent number of hit points worth of damage from someone else to yourself, or a single poison or disease, for 50 gp. This can result in the death of the person taking on the maladies if the effects are too great to bear.
Fortune: Gain inspiration (advantage on a single d20 roll of your choice) for 50 gp. This cannot be gained if you already have inspiration.
Insight: Gain a vision of something relevant to your immediate objectives for 50 gp.
Sanctuary: One or more parties drain a collective total of 25 hit points worth of damage as a blood offering (can come from an animal, captive, etc) at the shrine. For the next 3 days and 3 nights all participants must make a Charisma saving throw whenever they attempt to take violent action against another. If they fail, they find themselves physically unable to go through with the action, frozen in place.
Vengeance: Speak the name of a hated foe (or group of people who are sworn enemies of the deity), and gain +1d6 on your next attack roll against them for 50 gp.
Humanoid and Animal Sacrifices: Generally speaking, animals are treated as treasure for the purposes of sacrifice, using the mounts and trade goods entries under Equipment as guidelines. A single sheep is worth 2 gold pieces, whereas a mastiff is 25 gold, a riding horse 75, a cow 10, and so on and so forth.
Generally speaking, this makes the above amounts are rather costly for the lay worshiper: sacrifices are generally communal affairs, of weeks or month's worth of saving up enough money and raising choice cattle for when villagers and townsfolk really need the direct intervention of the divine.
As for sacrifices of humanoids and sapient beings, in most campaigns this is generally the province of evil deities and/or ones dedicated to battle and conflict (in the latter case, such sacrifices are restricted to prisoners of war). But regardless, deities only grant boons when a worthy subject is sacrificed. To count as worthy, the subject must have an equal or greater Challenge Rating to the person who seeks to beseech the gods. A 12th-level warlord can dedicate hapless peasants to death, but such displays are a trifling matter of no great consequence; far better to show respect and glory by capturing and felling a mighty adversary.