Sid Meier's Civilization V: Gods & Kings/Fall of Rome

It's the 4th century AD, and one of the greatest civilizations the world had known so far is on the verge of collapse. Rome had split in half and now struggles to maintain its influence, desperately fighting not only barbarian attacks from all sides but also its own internal cultural decline. For the barbarian tribes, it's a bright opportunity to sack a wealthy empire and establish themselves as the next great powers in the region. Meanwhile, the Sassanid Persian empire is in the midst of a golden age, enjoying its imperial glory while fending off its own barbarian assault.

Scenario details
This scenario involves 3 major factions: the Romans, the Sassanids, and the myriad Barbarian kingdoms. It is a score competition that lasts for 70 rounds, with the imperial civilizations earning points for controlling cities and the barbarians earning points for capturing them. Each faction is in constant warfare with the others, as such there is no diplomacy, science and religion is outright disabled too.

The Western Roman Empire and Eastern Roman Empire shares their score but manage their respective cities, with Western Rome taking the brunt of barbarian attacks. While they start with the most cities, they will quickly lose this advantage due to the overwhelming force of the other factions without exceptional play. The biggest weakness of the Romans is their social policy tree Roman Atrophy, which inflicts drawbacks with each policy taken. Thus their Culture output becomes cultural decline, and they are forbidden from selling off buildings, forcing them to keep producing Culture in cities. Even worse, each city lost to a barbarian incurs a lump of Culture, so their weakness can quickly snowball out of control.

The barbarian kingdoms comprise of the Celts, the Goths, the Franks, the Huns, and the Vandals. These disparate kingdoms each have their own scores and special traits, but all draw from the same policy branch: Barbarian Zeal, which provides excellent bonuses to military strength and city production. Additionally, barbarians earn extra culture by capturing cities and outright ignores unit maintenance.

The Sassanid Empire is a middle ground between the 2 other factions. They start with a fair amount of territory of their own and receive relatively less attention from barbarians. However, their own unique policy tree, Sassanid Legacy, doesn't provide as strong a bonus as the barbarians' branch, and their own cities provide more points than roman ones, which encourages a more steady pace of play.

Roman Atrophy

 * Opener: Each city of yours captured by a barbarian generates +20 Culture for you. Free at start.
 * Roman Heritage: +1 happiness for each city connected to the capital through city connections, -5% unhappiness from citizens in non-occupied cities. Free at start.
 * Barbarian Conscription: -10% Combat Strength.
 * Rise of Serfdom: Cities with Walls suffer -2 Production.
 * Neglected Infrastructure: Gold from city connections reduced by 50%.
 * Usurper General: +1 Culture for each barracks and a hostile army appears outside one of your non-capital cities. Requires Barbarian Conscription.
 * Popular Ennui: -1 Happiness from each Luxury Resource. Requires Rise of Serfdom.
 * Coin Debasement: +10% unit maintenance cost. Requires Neglected Infrastructure.
 * Dark Age: -20 Happiness. Requires Usurper General, Popular Ennui, and Coin Debasement.

Sassanid Legacy

 * Pahlavi Literature: +1 Culture per city.
 * Sacred Fire: +1 Happiness per city.
 * Mercantilism: +2 Gold per city. Great People born 25% faster.
 * Shahnama: Starts a Golden Age and a Great Engineer appears outside the Capital. Requires Pahlavi Literature and Sacred Fire.
 * Standing Army: +2 Production per Barracks. Requires Sacred Fire and Mercantilism.
 * Silk Road: +1 Gold for each Trading Post. Golden Ages last 50% longer. Requires Mercantilism.
 * Finisher: Convert all future Culture into Gold at rate of 3 to 1.

Barbarian Zeal

 * Opener: +20 Happiness, gain +15 Culture per point of city's Culture output upon capturing enemy city, no unit maintenance
 * Willing Recruits: +50% Production when training combat units.
 * Native Coercion: Build Courthouses 150% faster
 * Wily Veterans: Military units gain 100% more Experience from combat. Requires Willing Recruits.
 * Tribal Bloom: +5 Production in every city. Requires Native Coercion.
 * Destiny's Army: +50% Combat Strength for military Units which have another military Unit in an adjacent tile. Requires Wily Veterans.
 * Kingdom Come: +2 Food, Production, Gold, and Culture per city. Requires Kingdom Come.
 * Finisher: Convert all future Culture into Gold at rate of 3 to 1.

Western Rome (Honorius)

 * Unique Ability: Barbarian Enrollment - Western Roman Empire Legions will recruit defeated enemies to their side.
 * Unique Unit: Ballista - Replaces Catapult. +1 Combat Strength, +2 Ranged Combat Strength.
 * Unique Unit: Legion - Replaces Swordsman. +3 Combat Strength, can build Roads and Forts.

Eastern Rome (Theodora)

 * Unique Ability: Theodosian Walls - +10% Combat Strength if within 2 tiles of a Fort. Great Generals spawn 50% faster and provide an additional 10% combat bonus.
 * Unique Unit: Cataphract - Replaces Horseman. +3 Combat Strength, -1 Movement. Can receive defensive terrain bonuses and has only a 25% combat penalty against Cities.
 * Unique Unit: Dromon - Replaces Trireme. +11 Production cost, ranged unit with 8 Combat Strength, 10 Ranged Combat Strength and 2 Range. +50% combat bonus vs other naval units.

The Sassanids (Bahram V)

 * Unique Ability: Archaemenid Legacy - Golden Ages last 50% longer. During a Golden Age, units receive +1 Movement and a +10% Combat Strength bonus.
 * Unique Building: Satrap Court - Replaces Bank. +1 Gold, +2 Happiness
 * Unique Unit: Clibaranii - Replaces Horseman. +4 Combat Strength, -1 Movement. Can receive defensive terrain bonuses and has only a 25% combat penalty against Cities. Free Blitz promotion.

The Celts (Boudicca)

 * Unique Ability: Painted Rebels - +50% combat bonus when fighting against a civilization with more cities. Treats forests in friendly territory as roads. Embarked units have +1 Movement and pay 1 movement point to move from sea to land.
 * Unique Building: Cellidh Hall - +4 Culture, +3 Happiness, 1 Artist Specialist slot. Available way earlier than base game.
 * Unique Unit: Pictish Warrior - Replaces Spearman. +20% combat bonus outside of friendly territory, expends no movement points to pillage. +50% bonus to flank attacks.

The Franks (Chlodio)

 * Unique Ability: Regal Bloodline - Units ignore terrain costs when moving into any tile with Hills and move faster along rivers.
 * Unique Building: Mead Hall - Replaces Circus. +2 Culture.
 * Unique Unit: Seaxman - Replaces Swordsman. +1 Combat Strength, free Amphibious promotion, doesn't require Iron.

The Goths (Alaric I)

 * Unique Ability: Healthy Plundering - Combat units heal completely after pillaging a non-road improvement.
 * Unique Building: Hov - Replaces Market. +1 Happiness.
 * Unique Unit: Gadrauht - Replaces Swordsman. +1 Combat Strength, treat enemy roads as friendly roads, doesn't require Iron.

The Huns (Attila)

 * Unique Ability: Scourge of God - +2 Movement for all land combat units. Start with Attila, the Scourge of God. 2 Battering Rams awarded after capturing city for the first time. +2 Production per Pasture.
 * Unique Unit: Scourge of God - 6 Movement, heals additional 20 HP per turn on adjacent units.
 * Unique Unit: Battering Ram - Replaces Spearman. Acts as siege unit instead of melee unit, higher cost
 * Unique Unit: Horse Archer - Replaces Chariot Archer. +1 Combat Strength, no Rough Terrain Penalty, free Accuracy I promotion, does not require Horses

The Vandals (Genseric)

 * Unique Ability: Mediterranean Pirates - Naval units will "recruit" defeated ships to fight on their side.
 * Unique Unit: Axeman - Replaces Swordsman. +1 Combat Strength, free Charge promotion, doesn't require Iron.
 * Unique Unit: Trihemiolla - Replaces Trireme. +3 Combat Strength, free Supply promotion, can capture enemy ships.