First off, if you’re playing in my Fell Met Sea game please don’t read this yet. It’s 100% full of spoilers for my current thoughts on setting background that you haven’t learned yet. If you’re not playing Fell Met Sea, I’ve put together some ideas about how the previous civilization(s) that preceded my PCs’ present world fell apart. Check out the consequences of sacrificial blood magic!
Cultures of the world of the Fell Met Sea had been using small scale sacrificial blood magic for a long time before they broke reality with it. It was known to be a reliable source of power beyond what was normally available to a regular mage, and any group of collaborating mages could perform sacrificial rites to bind forces and perform specific tasks. I think it may be how they sped the construction of larger structures in their civilization, turning bloody power into bound entities that served as specialized tools (earth moving equipment, etc).
What was the breaking point that they reached, the thing that turned blood magic from a limited utilitarian tool into a door broken off its hinges and open to the dangers of the world? Resurrection? Building a new god? Consuming an old one?
An old god was no longer relevant to the people practicing blood magic, their rites and offerings unnecessary to the workings of the society. The researchers wanted to find a way to break down the obsolescent god and capture the energy it contained, perhaps to use it as a blood sacrifice of a sort… or to prove that its metaphysical death could be used in the same way that a mortal’s death could. Rather than risk letting all that energy go to waste, the researchers wanted to use that energy to open a permanent portal to a separate plane… maybe the elemental plane of earth or fire, in an effort to gain access to the resources therein.
Temporary portals were already known to be possible, and the blood inputs required were known and quantifiable. Those temporary portals had been used previously to access resources necessary for the construction of rare objects and modern cities. The energy theorized to be available from sacrificing a god ought to have been sufficient to open a stable portal for a tremendously long period of time—possibly forever.
Instead, the sacrifice was enough to crack the structure of reality and let consuming, infectious chaos from beyond leak through. This took the form of the gnoll plague, spread by the influence of the most powerful entities to force their way past the torn fabric of reality. It was only after the researchers’ ritual site and some of its environs were obliterated by magical kinetic bombardment that the sacrificed god’s energy was released and the portal permitting gnolls’ entry to the world was closed. Once the crack in reality had been opened, however, that was it; any use of blood magic thereafter risked small incursions of the fiendish influence which the god-sacrifice had brought. Blood magic was intentionally set aside, and few were willing to practice it further, or tolerate its practice by others.
Further calamities followed, as those who had relied on blood magic to fuel their society frantically sought other sources of energy. In an attempt to preserve as much of their civilization as possible, practitioners conducted their searches at a breakneck pace, achieving speed at the expense of caution. The resulting mistakes and catastrophes ultimately forced ever more desperate measures, culminating in an attempt to transpose their world onto another one to flee the consequences of their actions. Only half of their world came along for the ride, and few people survived. After the escape from their old planet to their new one, the use of blood magic was forgotten. Few of those who had known its workings lived through the transposition, and those who did refused to pass that knowledge on.