There are cosmic horrors – idiotic, ravenous, indestructible things – lurking on the fringes of the universe. They adore the taste of sentient life; it attracts them to the cracks in reality, and then they force their way through for a time, destroy/eat everything in sight, and retreat back to the madness-worlds that they call home. You know this for truth; its obviousness is inescapable. Most people cannot accept this at face value, however, and that is why we come up with theories about life-destroying comets and the like.
The old races – the really, really old races, the ones that the Elders of the Universe outlived – discovered, back in the day, the power of warning signs. Life magic, using the energy of expiring souls to create runes that, if not capable of destroying those horrors, at least made planets rich with life taste like moldy Limburger to them. (Metaphorically speaking, of course.) Benevolent members of those races explored the universe and found the prehistoric ancestors of all the alien races, and taught them the rituals – who to choose, how to kill and when and where, what to do thereafter. (Murder is simple. Sacrifice is difficult.)
Of course, people being people – even when they’re aliens – after a few tens of thousands of years, everybody forgot the why and how and when and where, and it just became ritual sacrifice to appease gods. Sometimes the gods existed; sometimes they did not. However, the existent gods didn’t complain, because sacrificial worship is still worship, even if it is messy. The nonexistent ones did not complain for obvious reasons. Eventually, of course, most civilizations grew up and left behind ritual sacrifice as a practice, because it was barbaric and cruel and pointless. And it was – the original rules to create the warding signs had long since mutated into uselessness.
On Earth, the old races taught half a dozen nascent cultures, all of whom managed to forget in different ways the purpose of their task. It’s not surprising that humanity forgot the reason for the sacrifices; what would be surprising would be if somebody managed to rediscover the practice and the need behind it. It would be virtually impossible, an act of archeology so amazing Indiana Jones would jump out of the fridge and doff his fedora in respect.
Which of course means that somebody does it. An accomplished archeologist pieces it all together, and then realizes that, according to the instructions left behind on stone tablets from a dozen ancient civilizations, Earth’s current set of warning markers is just about all used up. They should have been renewed a century ago; now maybe a couple of years’ time and one person’s knowledge are all that stands between the planet and eternal cosmic consumption.
The good Doctor only learns of this person’s work gradually, when other, less magically-inclined heroes ask him to consult on a grisly, occult-looking murder. Finding the person involved proves to be difficult, even for one of his calibre. But then comes the most troubling question: what if the archeologist is right? What if the heightened and honed magical abilities of the Sorcerer Supreme (and similarly powerful mages) aren’t an upgrade to the old blood magic, but merely an interesting, totally non-similar skill set, and they can’t deal with the cosmic horrors at all?
It’s Strange’s job to protect this reality from destruction. Does being the universe’s protector mean being the universe’s butcher when necessary?