First of all the Calabim of Sian
Decius is ORG/RAI. Org gives cheaper civics, helpful on higher difficulties, and cheap lighthouses and courthouses. It also allows a civ to build commandposts, giving an extra 2XP on military units and cuts build costs 20% (or 30%). Ok trait, better because of it being attached to the Calabim.
The Unique Units of the Vamps are along the melee line, Moroi (an axe replacement which can bezerk for blitz and extra strength and movement in return for a chance to turn barb or the bezerk effect to wear off), Vampire (the main unit, a champ replacement at Feud not IW, which can feast on cities for XP, on Morois for healing, and gift vampirism to other units. They also get access to Body I&II and Death I&II to give them real power. They are 1st weaker than their base counterparts), Vampire Lords (Immortal replacement, with access to the vampire abilities and all three spell levels in Death, Mind and Body. You need 12 promotions on a unit to upgrade rather than the usual six), and Brujah (a Zerker replacement which simply grants vampire level abilities and spells to the unit). Their hero is Losha Valas, a female vampire with some style. She comes at Fanaticism gets all the usual vampire abilities and gains Immortal any time she kills a living unit (unless she already has it).
The other big advantage is the Governor's Mansion, a courthouse replacement which only cuts costs 20%, but gives a hammer to every
face in the city. Big cities get really powerful production at Code of Laws. The lesser cost modifier can be a pain, especially as Decius isn't FIN, but the cheaper civics will offset the costs.
Mana given is Law (spells are Loyalty, Hosts and Valor, only level 2 is useful) which cuts costs 5%, Shadow (spells are blur {gets rid of defensive strike and first strike when fighting units}, Shadowwalk {gets rid of defensive strikes and ignores building defence when attacking cities) and summon mist form (invis 6str unit with shadow affinity), and Body (haste gives an extra movement, have to recast every turn, Regen allows extra healing when moving and after combat {basically march promo} but wears after full healing, and Graft Flesh, which allows the caster combine with another unit to make a highly promoted unit in many areas) which heals an extra 5% per turn.
I fully expect the strategy is to go to Feud, build pop and promote as many Vamps as possible before breaking out and killing us all off. If I were Sian I'd also look to AV for siege support, and I wouldn't go too much into the arcane line, despite the advantages of Shadow mages.
Yell0w as Rivanna of the Svarts
The big advantage of the Svarts is that they are blond, pasty-skinned emos, therefore for some strange reasons they can build improvements in trees without losing the trees (maybe its because trees are the only living things to be fooled by the threat of suicide when told to "clean your room" or "buy your own damned mobile credit" and take pity on them). So the Svarts will be another mid game power with access to extra hammers, and if they go FoL food.
Rivanna is ORG SUMM. Org is discussed, and Summoner gives the summoner trait to arcane units, meaning their summons survive an extra two turns, allowing for unit spam.
Their UUs are Illusionist which replace the Mage. Their summons are illusions, meaning that they can't kill other units, though with max damage set at 90%, a well promoted Illusionist can summon a unit to do max damage and then kill off the poor sap against him to become even more well promoted. The other is the Nyxin a reskinned (if awesomely so, why are emos so well treated in this game?). Their hero unit is Alazakan the Assassin who is, unsuprisingly, an Assassin. As a unit he is nothing special, but he comes with the Black Mirror equipment which is transferrable, allowing the holding unit to create an illusion of themselves to attack the enemy.
They get Shadow (discussed), Nature (discussed when talking about my picks) and Mind which gives 3% bonus to beakers (I think I said 5% above but civilopedia says I'm wrong) which gives the spells Inspiration, Charm Person and Domination, all of which I find to be weaksauce (I've never had charm person work in SP, is that typical?). Their final bonus is Sinister which gives a plus one attack modifier to their recon units, making them powerful in the field and compensating for the city attack malus when it comes to later units.
If I were a Svart I'd go economy early on, take Leaves for more economy, build recon for defence and explode in the mid and late game with melee units and arcane.
World spell is Veil of Night which gives HN to all units, making them look like Barbs to opponents.
Hiding Kneel as Auric
Do I really even need to go fully into the Illians by this stage? We all know about the Priests of Winter (and their high priest upgrades), the Ice mana, the no religion and Stasis (which now adjusts for game speed and gives a mini Golden Age to the Illians).
Auric is himself Cha, Arcane and Agnositc. Cha gives cheaper promotions and extra happy stacked on monuments, Arcane give quicker XP growth, movement 1 to arcane units and cheap mage guilds. And agnositc ensures he doesn't get religions.
They get access to Javelin Throwers, slightly cheaper archer units which don't require archery ranges. They also get temple of the hand, a buidling which turns a 5*5 square of land to ice, which the Illians can work as if it were normal non-river grassland. Manas available are Ice (slow, summon Ice elemental and fall of winter a damage spell), Enchantment and Law.
Strategy here is Priests of Winter, Mages and Swords. Attack in the mid game, and destroy your own land for others with the temples.
Techs for the three civs are Exploration for Illians and Calabim and Agri for the Svarts. Exploration is IMO the weakest of the starting techs, it doesn't give you a leg up in religious matters, to Calender (and agrarianism) or on the Mining>BW line, meaning that you're one tech further along the strongest lines in the early game than other civs.
Also interesting to note, we've all gone with civs which don't really show until the mid game when empires have matured and have a good number of strong cities. This will be interesting.
Travelling on a mote of dust, suspended in a sunbeam.