I think we should go ahead and assume that all this tech works more or less as well on the outer planes as it does on the prime. Saying things like "what if fission doesn't work on the abyss" isn't much fun in a scenario like this. Though a "limited tech" planar trait could be an intresting twist in a campagin like this.
Biological weapons = disease effect
Chemical weapons = poision effect
Nukes would have several components. The heatwave would be Fire damage and so the fiendish immunity should kick in. The shockwave should be plain vanalla physical damage (though I could see a case made for Sonic damage) and thus DR applies, though depending on how close you are to the blast I doubt DR 10 or 15 is going to matter very much (remember, the shockwave easily destroys buildings with hardness of 5-10). As for radiation; unless you want to invent a new energy type for it I submit that the existing energy type that most closely resembles the ability of full-spectrum hard radiation to just tear its way through anything is Force. The secondary fallout effects of radiation I would again treat as poision with some special rulse for templated mutations.
So Nukes would indeed be very effective, able to bypass or overcome most of the combatants natural defenses and probably dealing enough damage to kill most mid-tier and lower fiends outright (and dont' forget about massive damage checks for the big ones either). Probably more important than their effectiveness though is the fact that you can force a bunch of Expert 4 humans to build them for you and then strap one to the back of a Dretch or something and toss him into battle. Before you needed somebody willing and able to cast big-ticket spells to get that effect.
However, IMO even more important than the WMD's would be the small arms. Again, even mundane small arms could still have the potential to overcome low and medium DR. An automatic rifle in the hands of someone with the Burst Fire feat does 4d8 damage. A LAW rocket has a listed damage of 10d6, enough to make even a Balor take notice I think. Considering the tendancy of Blood War armies to rely on the "swarm of mooks" tactic I think the introduction of modern small arms would have a significant impact. Once again the major deciding factor here is that these weapons are absurdly cheap to produce compared to equavialent magic.
As for how this would affect the big picture of the Blood War. I don't think it would result in an instant-win scenario for either side. As someone else pointed out the same defenses that have always protected against buff-scry-teleport attacks will still work against someone attempting to deliver a strategic Nuke. Instead I imagine that the real impressive results will be on the battlefield, with that much firepower the fiends should finally be able to smash up the opposing army instead of just fighting to a standstill. Perhaps this really is the beginning of the endgame; perhaps the fiend's damage dealing ability has finally outsrtiped their ability to produce more combatants, perhaps you really can depoulate the abyss.
If so then both sides will seek to deny their opponent this decisive advantage while securring it for themselves. The source of the weapons will become the key strategic resource in this new pardagrim. Perhaps uranaium and nirtous compounds (gunpowder) really do only occur in significant quantity on the prime material plane(s). Similarily perhaps both sides want to conquer Mechanus for its mass-production capability. In the past the fiends have only relied upon their own breding pits for resources, now inter-planar supply lines are of paramount importance. If any mook who can pull a trigger is sufficent to be Blood War cannon fodder then both sides may begin impressing recruits from other planes into service in order to shore up their failing defenses; think of the opening scenes of Enemy At The Gates and the Russian defense of Stalingrad. Moreover, having realized that this is indeed the endgame, both sides are willing to disregard any treaty and forsake any allies in the persuit of victory. Sigil is no longer neutral. Meanwhile the other powers are realizing that the Blood War has finally spilled out into the rest of the multiverse. Is it too late for them to find a way to put the genie back in the bottle? If it isn't then that sounds like a cue for the PC's to enter. If it is then the question becomes what to do now? Can you contain the fiends and force them back to a mitual standstill? Or must the rest of the multiverse attempt to band together and destroy both sides before the Blood War overwhelms them? And what if they actually succede in that mad objective?
Or maybe the leadership of both sides forsees this and they decide instead to cease open hostilities and go all cold-war and MAD on each other. You thought conflics on the outer planes were resolved by proxy before? Let me tell you you ain't seen nothing yet. And Sigil just became the new Checkpoint Charlie.
However you cut it it should be prety cool. Good luck.
Biological weapons = disease effect
Chemical weapons = poision effect
Nukes would have several components. The heatwave would be Fire damage and so the fiendish immunity should kick in. The shockwave should be plain vanalla physical damage (though I could see a case made for Sonic damage) and thus DR applies, though depending on how close you are to the blast I doubt DR 10 or 15 is going to matter very much (remember, the shockwave easily destroys buildings with hardness of 5-10). As for radiation; unless you want to invent a new energy type for it I submit that the existing energy type that most closely resembles the ability of full-spectrum hard radiation to just tear its way through anything is Force. The secondary fallout effects of radiation I would again treat as poision with some special rulse for templated mutations.
So Nukes would indeed be very effective, able to bypass or overcome most of the combatants natural defenses and probably dealing enough damage to kill most mid-tier and lower fiends outright (and dont' forget about massive damage checks for the big ones either). Probably more important than their effectiveness though is the fact that you can force a bunch of Expert 4 humans to build them for you and then strap one to the back of a Dretch or something and toss him into battle. Before you needed somebody willing and able to cast big-ticket spells to get that effect.
However, IMO even more important than the WMD's would be the small arms. Again, even mundane small arms could still have the potential to overcome low and medium DR. An automatic rifle in the hands of someone with the Burst Fire feat does 4d8 damage. A LAW rocket has a listed damage of 10d6, enough to make even a Balor take notice I think. Considering the tendancy of Blood War armies to rely on the "swarm of mooks" tactic I think the introduction of modern small arms would have a significant impact. Once again the major deciding factor here is that these weapons are absurdly cheap to produce compared to equavialent magic.
As for how this would affect the big picture of the Blood War. I don't think it would result in an instant-win scenario for either side. As someone else pointed out the same defenses that have always protected against buff-scry-teleport attacks will still work against someone attempting to deliver a strategic Nuke. Instead I imagine that the real impressive results will be on the battlefield, with that much firepower the fiends should finally be able to smash up the opposing army instead of just fighting to a standstill. Perhaps this really is the beginning of the endgame; perhaps the fiend's damage dealing ability has finally outsrtiped their ability to produce more combatants, perhaps you really can depoulate the abyss.
If so then both sides will seek to deny their opponent this decisive advantage while securring it for themselves. The source of the weapons will become the key strategic resource in this new pardagrim. Perhaps uranaium and nirtous compounds (gunpowder) really do only occur in significant quantity on the prime material plane(s). Similarily perhaps both sides want to conquer Mechanus for its mass-production capability. In the past the fiends have only relied upon their own breding pits for resources, now inter-planar supply lines are of paramount importance. If any mook who can pull a trigger is sufficent to be Blood War cannon fodder then both sides may begin impressing recruits from other planes into service in order to shore up their failing defenses; think of the opening scenes of Enemy At The Gates and the Russian defense of Stalingrad. Moreover, having realized that this is indeed the endgame, both sides are willing to disregard any treaty and forsake any allies in the persuit of victory. Sigil is no longer neutral. Meanwhile the other powers are realizing that the Blood War has finally spilled out into the rest of the multiverse. Is it too late for them to find a way to put the genie back in the bottle? If it isn't then that sounds like a cue for the PC's to enter. If it is then the question becomes what to do now? Can you contain the fiends and force them back to a mitual standstill? Or must the rest of the multiverse attempt to band together and destroy both sides before the Blood War overwhelms them? And what if they actually succede in that mad objective?
Or maybe the leadership of both sides forsees this and they decide instead to cease open hostilities and go all cold-war and MAD on each other. You thought conflics on the outer planes were resolved by proxy before? Let me tell you you ain't seen nothing yet. And Sigil just became the new Checkpoint Charlie.
However you cut it it should be prety cool. Good luck.
