Todd Kenreck: In Mordenkainen's Tome of Foes, there are a lot of adversaries, and some of them are not from the prime material plane. That brings us to the Blood War, the conflict, the fight between devils and demons. And oddly enough, this war may be keeping many alive in the D&D multiverse.
Mike Mearls: So the Blood War is probably the most ancient of all the struggles, the conflicts that this book talks about that we see in Mordenkainen's Tome of Foes. Demons and devils ... demons are Chaotic Evil, and devils are Lawful Evil. Now, you'd think, "Hey, they're both evil. They share something, so maybe they'd come together." But no, one of the key traits of evil is this sense that I come first, and so it's very hard for evil factions to work together, at least for a long period of time. So maybe at one point they worked together. Now, they definitely don't.
And then, you have law and chaos, which are very much ... the devils would love to control the demons if they could. The demons just want to tear apart Hell and claim it as their hunting ground. And then, we also have the further complication that demons and devils, unlike other planar creatures, don't have the freedom to use magic to just teleport or gate themselves from Hell to anywhere else. They are much more restricted. They have to be called. They need someone outside of the Abyss or the Nine Hells to call them forth.
Now, if someone was to foolishly just build a gateway that was open, they could just walk through. But they don't have anywhere near the same flexibility that other planar creatures have to travel, with one exception. I mentioned they could just walk from one place to another if someone built a gate that stayed open. Well, in the lower planes, that gate is the River Styx. The River Styx wends through a variety of the lower planes, including the Abyss and the Nine Hells.
So demons naturally travel along the banks or try to sail along the Styx, which is very treacherous, but you can manage it if you're powerful enough or clever enough. And they naturally attack everyone they come across, and Hell, the Nine Hells, is a very tempting target for them. So the demons and devils fight all along the Styx, any plane that touches it, any areas around it. The demons just simply seek out devils for the thrill of destroying something else and claiming more territory. The devils are primarily interested in defending the Nine Hells, in defending their realms, and when they can, making forays into the Abyss to just try to burn out the infection at its root.
Now, that just seems like a very simple two sides that don't get along fighting. But what Mordenkainen would tell you is that this is the root of the balance, that evil is obviously a very powerful force in the cosmos in D&D, right? Three of the nine alignments are evil, opposed by the three good alignments, with the three neutral on the liminal. Mordenkainen, in his study, would tell you that if one side, the demons and the devils, one of them was able to win the Blood War, that would be an apocalyptic end to the universe.
Asmodeus would be so powerful, claiming two entire ... basically two entire planes in the entire planar structure and so many resources and so much power that no one would be able to stand against him. He would impose the hierarchy and bureaucracy of Hell on everything, the outer planes and the inner planes. Demons, on the other hand, if they were able to overrun Hell and claim it as their own, would similarly ... the demon princes would grow so powerful that, as they just bashed against other planes and sought pathways into the inner planes and into other outer planes, they would overrun the universe.
So in some ways, it's cosmic luck that these two factions are locked in combat, but neither one either has the organizational capabilities to say, "Let's stop fighting," like the demons. They're chaotic. All it takes is one demon lord to decide he wants to try to invade Hell or just demons wandering down the Styx and attacking things, which then puts Hell ... which would sue for peace, potentially, if they could. But they can't, because who would they ... They could create a peace treaty with Orcus that doesn't happen, and nothing stops Demogorgon or anyone else from attacking. So they're locked in this endless struggle, and the universe is all the better for it.
Now, what this means, though, is that this struggle spills over everywhere. You'd think, "Hey, two evil factions fighting. This is great. It's going to keep them busy. It makes things easier for the good guys." In fact, it has the opposite effect, because anywhere you have demons, you'll soon have devils trying to thwart them, and vice versa. And on top of that, since both demons and devils are really driven by claiming souls from the material plane of mortals, both of them have very aggressive cults in the inner ... in the material plane in various worlds of D&D that seek to subvert mortals, claim artifacts, claim secrets.
If someone could find a secret that allowed the demons or devils to move freely across the planes, that would end the Blood War in one side's favor or the other. So just the raw aggression that both sides bring means that every world of D&D is in danger and that demons and devils would happily scour them looking for an advantage, because they know the stakes are that high, and they want to win. And they'll do anything to achieve victory.
It's raged back and forth. I mean, the history is as old as the cosmos, and so it's raged ... it's waxed and waned, but it's always been there. And in some ways, it's become such a monotonous part of the cosmos that many sages just feel it's just part of the mechanism of the world, of the planes. There's always going to be this stalemate, and it will never change.
But that's where Mordenkainen, other adherents of the balance ... to them, the Blood War is the centerpiece conflict of the cosmos. It's the most important one. It's the one that if it fell out of balance would spell doom. And so he and his associates and his allies who are interested in the balance are very much ... they very much keep a careful watch on how the Blood War's progressing, because luckily, we haven't had any decisive moments yet.