I understand how it works and what effects it has I just don't really understand why any civilization wants it. From a purely strategy point of view I don't see even why the Sheaim would want it. My Sheaim civ was wiped out by the horseman in about 15 turns with a 85 counter. Also on an unrelated note are there other units that can be permanently summoned besides skeletons?
I've just managed an Armageddon victory (a Conquest victory where the Horsemen do most of the work) with the Sheaim, but I think I was very lucky.
It's essential that the Horsemen appear nowhere near your cities. They can actually be survived, although they might destroy half your empire in the process. The real problem is that you want them to destroy the rest of the civilisations before they get to you.
Playing the Sheaim I spammed Elegy of the Sheaim and did everything I could do to increase the AC. I switched off research once I had what I needed – which is high AC, Ashen Veil founded, Corruption of the Spirit (for Rosier the Fallen) and Infernal Pact. With research off I was able to buy Meshabber of Dis in one turn once the AC hit 70.
Four Elegies timed in four cities brought the AC to 93 (+3 from Meshabber's creation), causing Wrath.
The Horsemen didn't appear near to me, and with Wrath on and Worldbreak cast soon after, the Horsemen destroyed everyone in the world except for me (the Sheaim), the Infernals and two civilisations lucky enough to get their own island each off the main continent.
Ars Moriendi was the first to come my way, and I don't know how bad it could have been – his early death was a fluke. Buboes arrived next and destroyed two cities before I threw every unit I had in a city's Planar Gate at it and it fell. Stephanos and Yersinia fell to Rosiel and Meshabber respectively.
After that, the key advantage of the AC was that it no longer mattered what happened to my cities, or even how many I had left. Meshabber of Dis destroyed Hyborem, Varn Gosam and Dain alone.
…in short, get the Meshabber of Dis ASAP – his only requirement is AC, not technology (not beyond Ashen Veil). Horsemen + Wrath + Worldbreak + Meshabber is the end of the world.
Oh, and you need Tar Demons. Planar Gates will produce them in large numbers once you have a Temple of the Veil. You don't need any deviation from your technology route (just Corruption of the Spirit), they have 7 + 1 Unholy defence and are immune to Death, Poison and Unholy damage. That is, a Tar Demon fighting Ars is 13 vs. 7 instead of 29 vs. 6 (a Champion – that unit your enemies might not even have yet).
The Sheaim are
brilliant at high AC, but only if you spam it early, found Ashen Veil and are actually willing to watch the world burn.