I think you just think that this very rare case is the same like your usual games which is also my usual games, because you clearly state many things that happens in my usual games.
If you are so inclined to know every detail of my case, ok, i'll explain detail by detail.
So wait you're telling me that 4 AI had armies, that you usually see only on late game AI even one them, with perks that you usually never see on AI more than one at a time, they all perfectly concaved those armies on your borders at the same time, which you didn't notice, and they all pretty much attacked you at the same time which you did nothing about (like say paying off one of them when they demand something) and all this by turn 40? And you had what against that? Also did they have several perks or not on all of them? Cuz you mention protection amulets from gems (a rare resource most of the time, so they really can't all 4 have it) and silver brewery against spells (which of course you cant aim at the units without the perks, cuz that wouldn't be sporting, right?) and they also all seem to have silver weapons and maybe two armor upgrades too? At turn 40? With 25+ units a piece? On four different AI? Do excuse me if I think you're slightly full of horse puckey.
Also for the record, I did say unit aoe. Like you know what the court werewolves use. You know the ones you most likely used to get those wolves of helia you mentioned having earlier. Which also have cleave by the way. Those are really spiffy for killing of weak but numerous units. like say healers and cutthroats. If they're all that clumped up, then a pair of court wolfies will kill off 9 enemy units per cooldown, even assuming you don't use terrain bonuses. If you do catch them on lava or swamp, you can one shot those 9 in one casting. And yes, waging a war of attrition means you lose cities. Specifically you're going to let them go to low hp and then raze them yourself to deny them from enemies. Preferably with a throwaway unit like a spearman inside to give the city extra power, assuming you have a couple to spare. That's kinda the whole point. You make them lose 3 or 4 units for every one you lose. And honestly, the cities you lose will be pretty easy to replace in this game. Your capitol will be awesome for defense. That's what the aoe is there for. Smack a unit with regen like a werewolf or a troll inside, and it'll take them like 10 turn to kill the city off even with full surround. Which by the way you shouldn't be allowing in the first place. At which point, they should all be dead anyway.
First things first, each player have their own rules, in my case, no diplomacy, no demand, no asking anything to end war, that is one of my rules. So i won't pay anything to end the war, what i'm after is epic war. Basically i could've stopped all 4 war easily if i bribe them or agree on their demand, but that against my rules, and, in all my impossible games, i never saw AI muster such amount of troops in turn 40, in most of them it's just 1/3 or 1/4 of it, with much less perks.
And no, they don't concave it at the same turn, first it's just one AI, and at turn 40, like i said, i just spotted a unit, bam, you discover another great mage message pop, then i end that turn, that GM ask for a war, in my rules, i must accept it, even if the price they demand is very irrelevant. In all my other games, i win easily, with any kind of strategy. Then next turn i got swarmed by the number i mentioned, still i can take on all of them. The 2nd GM is discovered right at the same turn the 1st declare war, the next turn he follows, the next 2 turn 3rd and 4th AI is discovered and at the same time the 2nd AI declare war, again i bribe it because it's against my rules and i still can beat both of them, then next turn the 2nd AI swarmed from above, i can still win but the next turn 3rd and 4th AI begin amassing in other side of my empire, i knew they will declare war on me, at first i thought only 2 AI that has such number, but i was wrong, they too had the same number. Turns out i was in the center, i suspect that those AI originally intended to use those troop to war with other AI, but they seem to discover me simultaneously, and the broken diplomatic thinking of the AI made the 1st declare war, by my code i must not reject, then the next follows its footstep, by turn 50, they already swarming.
I had what against what? i had only iron and magic node for resource node, ok well, i have 1 pig, and 1 gold too, and 1 oly site. 2 AI has silver, magic node, nevril, adamantium, gem, the other 2 had similar resource plus labyrinth, minus adamantium and nevril. Gem is rare yes, but do you read that it happens 1 out of 50 game in impossible difficulty, it is possible to happen that all of them have gem, actually the number of gem in huge map is varied, i've seen a max of 6 node in original world (ardania) in one of my game. In this one, i didn't know what is the maximum number gem node in ardania, but 4 of them that assault me had it. Yes, they (all 4) do have masterwork and fine armor, and both silver weapons and silver brewery, which never happens in all my game except this one, AI lack the ability to take advantage of combining race specific perk (silver weapon+brewery), even in all my game except this one they never have both silver perks at the same time at single unit.
As i've mentioned it, if my WoH or old troll attack, in the next turn they will be gone, i even tried it because i don't trust my calculation, then i'm forced to reload the save which against my rules (but i ignore it this time because i want to see how it turns out in the end), 1 unit of an AI can hit approximately at 8-10 (clerics and cutthroat can reach 11-14) damage without crits, with such a swarm, i'm forced to use defend, also they even manage to kill my old troll from full hp at defense state in 1 turn (20 hp regen is useless if it is killed in 1 turn), because they also have ranged unit, if they only have melee unit, only 6 melee units can attack a unit, but the case is different with ranged unit. Now, how often AI can kill an old troll in 1 turn, it never happens in all my games, just this one which is just 1 out of 50 ish, so it's fair enough imo. But is it impossible for just 1 AI to kill an old troll at defense mode in 1 turn if they have variety of units (melee+range) that occupy all hexes at 1360x768 without any zooming in and out? No, it is very possible, though i only experienced it 1 out of all my 50 ish games on impossible, this proved to me the other side of the AI.
Actually that is indeed a war of attrition, but will you sacrifice a city against 100 ish units that is already at the perimeter of you empire at turn 40-50 and all of them march to destroy it, that is why i said that it is not a war of attrition anymore, it is genocide, but do note, that this is also a rare case, because it never happens in all my games that those AIs strike in such unison.
Actually, i was as shocked as you (if you read my first post), all those things never happens in my other games with identical setting, even until now i'm still trying to reproduce such situation to no avail. My reason of sharing it with all of you is simple, to point out that such a state exist, in AI programming, the many myriad path of choices of AI strategy is like a family tree with many branches, each node/leaf have value, worst case, normal case, or best case, the cases are influenced by many factors in the game, my point of sharing this is that the best case met our expectation, it's just extremely rare to happen.