Actually, believe it or not, Stephanos is the strongest of the 4, (In some situations). Sure, in comparison with the others, he is way behind, but, remind that he comes first.
Most probably is that is early-mid game when AC reaches 40 (If some Sheaim has been working on that, or something else).
That does mean that Stephanos is stronger, very strong, mid-early game since players do not have the ability to counter him with powerful units, mages or anything.
In contrast, usually, when the AC reaches 50 or 70, it's usually late game. The riders that come then are, indeed, stronger than Stephanos, but however, players in that moment do have late game units that, with some effort, could kill those Riders easily. (Also mages with summons, etc.).
Another thing that makes Stephanos OP is that his mere presence creates 5 unhapiness in ALL cities in the world (Excep those that are at peace with Agares). Early game and mid game those effects are terrible, most probably due to lack of happiness buildings. (Sometimes it comes also with the Blight effect, because the AC increased dramatically by something, causing many AI and player cities to starve).
And that's not all: Should Stephanos defeat a unit, that unit has a big chance to being dominated by Stephanos, adding more paing to the situation.
And the others? Buboes does raises the AC a bit when he kills a living unit (Due to his sword) and has an ability that can give Burning blood to units (Although Burning blood does have a chance for your units to become barbs, it can be easily countered with a loyalty spell, and also, Burning blood enhances combat) . Nothing else. Not more painful that Stephanos isn't it?. (This means that Buboes is weaker than Stephanos early game!). Also it should be noticed that Stephanos have more base strength than Ars and Yersinia, though less than Buboes. (The others have more Strength in form of Poison and Death, so if you are immune they're weaker than Stephanos and Buboes).
Yersinia's unique ability is to pass the plague to units in combat. It hardly matters since Yersinia will kill weaker units most of the time. (His worldeffect causes unhealth..). If you're resistant to unholy or Poison damage, you'll have a much easier time with Yersinia than the others.
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Ars is somewhat stronger than Stephanos, but most of his power relies on death and unholy and his base strength is not better than a Phalanx. Though his special spell causes a lot of damage and Ars can create a powerful Wraith if succesfull in combat, Ars is pretty much late game Rider since he comes at AC 70 (I think?) so a well prepared army wouldn't have much trouble at all.
"fun factor" issue. Stephanos is not "fun" in that incarnation. He should represent a very visible threat and be nasty. But, not something that you can't kill due to game mechanics and "special abilities."
That's why Stephanos should be revised. I'm afraid this is not a bug report anymore, but is better here than there and set up the theory again.
I don't think the riders need to be nerfed, even if Stephanos is only the first of the riders, he comes at AC 40, which means 40% Armageddon. I doubt that a RL nuclear world war would be as bad as 40% Erebus Armageddon.
I do however realise that the current Rider behaviour is far from perfect: Even if it is the most efficient way to destroy all of civilization, 4 Riders sitting on the same tile in the middle of the ocean and doing nothing is neither fun nor flavourful. Would it be a lot of effort to create a Unit AI which will always attack - even if the odds are against it? Kind of like an even more aggressive version of Crazed?
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Right there, they shouldn't be nerfed (...), but (sic) they have to be revised. After all they represent the Armaggeddon and Hell coming to Erebus, isn't it?. They're supposed to create chaos, but at the same time, become a part of the game, without spoiling it badly to all players.
