I'm seriously considering having the Baatezu overun much or all of Acheron in my next adventure, but I wanted a good reason to present to players of why this is happening.
Or more to the point. Why hasn't it happened in the past ? The armies of Avalas seem to be vastly inferior to those of Baator in strength, size and organization.
The material I own doesn't present a credible reason justifying why this hasn't happened.
I would like to ask your opinion on why Baataor hasn't all but taken over Acheron.
A) They can't spare the manpower from the Blood War and elsewhere. Even with most petitioner and other armies not packing nearly as much of a punch as the Baatezu (though some weight needs to be attached to the fact that many of those armies are headed by powers, some of them greater), it's certain that it would take quite some time and effort to control Avalas, and the Baatezu are not exactly the most numerous of exemplars. Thuldanin may present its own unique set of challenges (unsure at the moment of whether outsiders are subject to the layer's petrifying effects), and claiming Tintubulous and Ocanthus pretty much means sending a great deal of troops to control a pair of gigantic wastelands with dubious strategic (let alone tactical) value. In addition to this, it's said that Acheron has fewer petitioners on it at any given time than than there are souls bound for it, with each petitioner "standing in line" until it is allowed to enter by the death of a previous petitoner. So no matter how many times they destroyed every army on the plane (and it needs to be noted that any individual army wiped out would strengthen those remaining), more would simply spring like mushrooms from the ground.
B) Their efforts would soon attract attention and opposition from most every other major player in the Multiverse since it would be tinkering with the status quo: the Celestials and Modrons would be opposed for sure because of allegiances to Good and/or Law. The Yugoloths probably don't want to see the Baatezu do with Acheron what they themselves did with Gehenna (although they might go along for the ride if compensated, or at least pretend to...), and the Rilmani probably would regard it as an unacceptable alteration of balance. The Tanar'ri would attack them simply because that's what they always do, and the Slaad just might show up to add on the mess. Unknown how many non-native powers might join the cause against them, but some, like Clangeddin Silverbeard, might very well. In addition to this, it's fairly certain that the Harmonium and the Fraternity of Order (likely also the Order of the Planes-Militant) would show up to pull the Mercykillers out of the fire, and that would mean alienating three of the largest and most influential mortal groupings likely to hold some sympathy for the Baatezu cause in other matters.
C) Every time the Baatezu would conquer an area of Acheron, it would soon slide off into Baator. With what I in my previous post called the Malthusian laws of petitioner presence, likely this would only mean that Acheron would continue to exist in a different shape but effectively much as it had previously been every time it happened. Morever, if an actual layer of Acheron slid off into Baator, that will create its own set of problems: what if there might suddenly be Ten or Eleven Hells instead of Nine?! The Baatezu seem like just the sorts to have a profound worry for such matters, and if it meant the potential rise of new Lords to add to the power playing and intrigue of Baator, most of the current Lords of the Nine would be tremendously uninterested in it coming to pass, and might even seek to stop it from taking place. The alternative to slide would be the Baatezu taking on the more Neutral-tinged Lawful Evil of Acheron, which is equally unacceptable in their eyes.
D) Who is to say they already aren't trying in the only way they currently can? I always did interpret the fight between Maglubiyet and Gruumsh as at least in part a proxy war between Baatezu and Yugoloths (judging by who gets to be a high-up in each army), waged for some unknown reason...
However, a faction of Modrons who are slightly tainted by evil have just recently moved into Acheron. This could actually cause the devils to take more interest in the plane, at least to counter the modron infestation, or even to build bridges to the evil(ish) modrons and try to get them to slip further toward evil.
However, I think Estavan nailed it when he said their Blood War consumes all their resources. It would be too much of a drain on them to wage a war on two fronts.
To conquer (part of) Acheron, the Baatezu would have to become another one of the many armies fighting for control of the plane. Either that, or the part of the plane would, as suggested above, be changed enough to make it slide into Baator.
I do not think war is the way to conquer a plane anyway, as much as the fiends might wish it to be different. Isn't belief that which makes a difference on the planes?
On another note. What exactly is the difference between Acheronian Lawful Evil and Baatorian Lawful Evil.
I usually picture the usual acheronian trooper as someone who could easily kill hundreds of people if ordered to without any pity, mercy or regret while his/her face shows little if any expression.
How is this level of carelessness about life and displayed brutality (If I'm right) any different from the baatezu ?
Acheron is the loss of identity - no, the surrender of identity to groupthink. It's orders for Order's sake. It's clans and nations and ethnicities and in-groups and out-groups, it's prejudice and bigotry and us and them and feuds and rivalries carried on long after they lost their meaning, purely for the sake of tradition and custom and dogma and because we're us and they're them. It's wars with no causes, war for no purpose other than war, it's the darker side of bureaucracy and group identity and patriotism and jingoism, it's when you think of yourself as part of a group and that starts to become dangerous, because you can't see yourself anymore.
Baator is Law and Evil in perfect balance. Where Acheron is evil, it's only for the sake of furthering Order - Order comes first in Acheron, before anything else, and evil is only a tool for furthering Law.
In Baator, Law exists for the sake of Evil and evil exists for the sake of Law and both exist for Evil Law, malevolent order, ordered malevolence, punishment not for the sake of order but for the sake of suffering... and suffering for the sake of order. Baator is out-and-out tyranny, an iron-tipped boot stomping on your face forever. It doesn't just want to assimilate you, like Acheron does - it actually hates you, not because you're in part of a hated group but because it hates everyone, especially itself.
In Acheron, at least, there is fraternity and fellowship within a unit, within a platoon or dragoon or phalanx or family or clan or faith or science or creed. Those who know their place in Acheron's system are rewarded and accepted and even perhaps loved - hate is focused outward, toward the outgroups.
In Baator there is no love, there is only the ice-cold machine that grinds you up and spits you out and manufactures more of you to throw away for the sake of glorious spiteful waste. It's a machine not for just the sake of being a machine, like in Acheron or even Mechanus and to some extent Arcadia; it's a machine because you will hate being part of a machine, and it relishes your hate and disgust and terror and feeds upon it and licks its lips while crushing you in its iron jaws.
Acheron is a heartless machine created with the purpose of furthering Order. Baator is a metal machine monster that will eat out your heart, replace it with another monster, and encode your entrails with your personal data before dropping it all in the shredder and burning in whatever it damn well likes.
Acheron uses evil as a tool for furthering law. In Baator, law is evil and evil is law.
In Gehenna, law is only a tool for furthering evil, to be used or discarded as they see fit. It's so much worse than Baator ever was. The Gray Waste is worse still.
In Gehenna, law is only a tool for furthering evil, to be used or discarded as they see fit. It's so much worse than Baator ever was. The Gray Waste is worse still.
That is a bit subjective...
I mean really, what is the difference between being ripped apart by a Mezzoloth or tortured by a Chain Devil?
Would you rather have some Pain Devil rip your soul apart so you can become a quivering lemure or some Yugoloth using your soul for a foul experiment?
Not one of those planes are hospitable in my opinion.
That is a bit subjective...
No, Gehenna has more emphasis on pure Evil than Baator does, which divides its energies evenly between promoting evil and promoting law.
This isn't subjective; it's what defines the plane, and what the plane defines. Gehenna is more evil than it's lawful. Baator is equally lawful and equally evil. Acheron is more lawful than it's evil. The Gray Waste is entirely evil, and neither lawful nor chaotic (or equally lawful and chaotic).
As for your questions, the difference is that the mezzoloth will tear you apart because it hates you, while the kyton will tear you apart both because it hates you and to enforce order. This is actually an important difference, because if tearing you apart is unlawful, the kytons won't do so. The mezzoloths, on the other hand, may well break the law if it suits their purposes, as tearing you apart generally will. Therefore, Gehenna is worse, since the chances of getting torn limb from limb are higher there.
The only difference between Gehenna and the Gray Waste in this instance is that in Gehenna, the mezzoloths will use the law to justify themselves if they can, while in the Gray Waste they won't even bother to do that much. The Gray Waste is worse than Gehenna in other ways, since it sucks you dry of spirit and will while you're waiting for the mezzoloths to arrive. Protection from fire and heat can save you from Gehenna's environmental fury, while avoiding the Gray Wasting is much harder.
Avoiding discussions on what brand of evil is worse, I think the reason the Baatezu have not shown a great interest in Archeron has already been mentioned. Simply put, Archeron is more trouble than it is worth. The Tanar'ri aren't going to be invaiding via archeron, there is not much in archeron the baatezu want or need, and ******* off the locals could be costly.
Really, it's the same question as "why haven't the Baatezu conquered Plane X?" Where X can stand in for virtually any place in the multiverse aside from the Abyss (too vast), and, possibly, the Ethereal (too dispersed), Limbo (any center created will not hold), Mount Celestia (outside chance it might be too well defended), the Inner Planes of Negative and Positive Energy (too uninhabitable), and the Prime Material Plane (too vast). If they put their minds and the full bulk of their numbers (the caveat here being that I have gotten the impression that there are meant to be a good deal many more 'zu than 'loths, even as neither are particularily numerous) to it, they might well be able to overrun any plane and bludgeon the locals into submission (or death). The real issue here is that once they did, there'd be little else they could do after, quite likely not even defend Baator due to the drain of forces needed to guard their new holdings.
It's not clear if any of the other Exemplars (the Tanar'ri being the ever-academic exception) would be able to do even that much to begin with, given their likely numbers.
They do have the power to do so.
All that is missing is a compelling reason to do so.
The Tanar'ri aren't going to be invaiding via Acheron.
Well, they could. They could use the portal in Plague-Mort to enter the Outlands, move their army across the Land, enter Acheron through the Rigus gate, and from there invade Baator. But it would be extremely difficult (the Rigus gate is deep underground and heavily defended, and the Outlands itself can be hazardous), and after they did it once they would lose the element of surprise in future attempts.
Might be a good gambit once, though, if they could pull it off.