Here’s a spell for your standard evil ritual, performed by maddened (or rapidly maddening) devotees of dark deities. Sometimes what you want to do with your magic is possible in theory, but impractical or impossible due to the cost. Well (goes the reasoning), if your need is great enough, why not offload that cost to others? All you need to do is scream the Altar-Chant’s invocation, let the blood out of a few victims, and you’re in service and can get on with summoning Ycnàgnnisssz or the Nameless Mist or what-have-you.
The altar-chant is a brief ritual. The base difficulty is d8, and the cost is not strain, but rather the permanent sacrifice of one point of stability. For as long as concentration is maintained after the Altar-Chant is completed, blood can be let from victims and the damage (enough to fill the victim’s HP meter) used in place of the strain or fatigue that a spell or ritual would normally demand. It is theoretically possible, with an appropriate medical skill check (d10), to limit the blood loss so that a sacrifice isn’t killed in the process. If none of the sacrifices die, the caster’s stability loss is temporary rather than permanent.
Using an enchanted altar and blade, or similarly specialized tools, decreases the difficulty of the Chant by one step. Extending the length of the Altar-Chant to one hour per victim doubles the amount of “energy” that can be harvested from each sacrifice. Repeated bouts of sacrifices will have a deep and lasting impact, over time, on the local nature of the Shadow, among other effects.