Basically, there is this huge campaign I am making. I intend it to be world-shattering, apocalyptic, "nothing is the same anymore" type thing.
The Tower of Incarnate Agony is completed, with the dead god used being Aoskar. From it, the General of Gehenna uses the Heart of Darkness to "purify" billions of Gehreleths, transforming them to Yugoloths. Their chaotic taint being expunged results in the Spawning Stone generating Slaad non-stop, similar to how the original use of the Heart of Darkness created the Obyriths and Ancient Baatorians. Using his now extremely bolstered army and the Crawling City, he interrupts a Blood War battlefield, demanding that all fiends bow to him or die. After immense battles, the two warring fiends finally agree on a temporary truce, fully intending to restart the Blood War after the united forces of evil conquer the Outer Planes. Meanwhile, the Sign of One succeeds in resurrecting Aoskar, who is now tainted by the Tower of Incarnate Agony. The General of Gehenna offers Aoskar dominion over Sigil, if he helps them. Aoskar creates a portal to Sigil, the fiends overrun it, and it's up to the players to stop them.
Now, I doubt this would be good a lot. I doubt the players would be able to fight the General, even indirectly, I doubt that it wouldn't change things too much, I doubt that such things could happen without the Lady of Pain instantly flaying everyone, I doubt that it'll fit in with Planescape, I pretty much doubt everything about this idea. But I really find it awesome. The whole thing is, I don't just want to have this be a single sentence of fluff in my Modern Planescape's background which is "The General of Gehenna temporarily stopped the Blood War, nearly conquered Sigil but was flayed, and now Anthraxus is the ruler of the Yugoloths".
I believe the worst critic is the self, because it is either too soft or too harsh on his work. So that's why I came to you.
Yugoloths do have enemies who effectively counterbalance their evil influence. Otherwise they would have taken over the planes long ago.
The celestial races would be obvious candidates. Even though they don't 'crusade' all that much in the Planescape setting, they might easily do so if the yugoloths were to make a major move like this. (It wouldn't hurt if a group of PC adventurers were to petition good-aligned outsiders for aid, either.) Both the tanar'ri and the baatezu would definitely chafe under the yoke of the 'loths, and PCs (or even celestials...) might take advantage of this, stirring up mutinies or getting baatezu/tanar'ri troops to perform only in the most minimal and token fashion possible. The rilmani might also easily get involved in countering the efforts of the yugoloths.
Since the General of Gehenna not only attained his position but has held onto it so long, perhaps he has a very solid understanding of his own limitations. In other words, he's canny enough to know he'd be flayed if he tried to conquer Sigil by force himself. (I'd imagine other fiends would have tested this idea at some point in the past...) He might use cannon fodder (i.e. tanar'ri and baatezu) to attack Sigil if he has a particular goal in mind. There are also mortal pawns. Powerful mortal NPCs, perhaps a factol...
I really like the idea of a resurrected, evil-tainted Aoskar, but yugoloths really don't care for Powers, and I don't think they would knowingly work to increase the power of any deity. Even so, they might not refuse to work with deities altogether (they certainly did in first edition...). If the General offered great power to Aoskar, he would only do so with the full intent of pulling the rug out from under the Power later.
The General would have so many enemies at this point, including immortal ones, that a handful of mortal adventurers aren't likely to be his first priority.
Also, simply killing off a group of mortals isn't necessarily something an archfiend would find ideal, since (assuming their alignment doesn't have an 'E' in it) it would only send their soul-energy to planes that are NOT the Grey Waste. Much better to corrupt them instead. If the mortal gadflies are really that much of an irritant, he could detail one of his subordinates to focus its efforts on finding out their weaknesses (avarice? pride? lack of compassion?) and turning them toward Evil instead. (If even partially successful, this would have the pleasant benefit of causing the PCs' celestial allies to reject them.)
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!