While a core part of the Ages structure in are the four main Legacy Paths and their powerful bonuses at the start of the Exploration and Modern Ages, there are also unique Dark Age bonuses that can only be acquired by failing to complete any of the milestones in a given path. These Dark Age bonuses are distinct in that they provide powerful advantages but also include a penalty of some sort. These Dark Age bonuses can be planned for and used strategically or may come as a lifeline if you find yourself in dire straits.
Dark Ages are only awarded for the Antiquity and Exploration Ages, as there are no Ages to apply a Dark Age bonus to after the Modern Age (at least for now). These bonuses have distinct use cases and will not be helpful in all situations, but they can also allow you to "" if you have fallen significantly behind other civilizations. Knowing what each Dark Age does may come in handy if it's necessary to pivot during an Age transition and offer an alternative if no Golden Age bonuses are available.
If you fail to complete any milestones in any of the four Legacy Paths, a Dark Age becomes available at the beginning of the next Age. Only one Dark Age or Golden Age bonus can be selected, and each Dark Age includes a penalty of some kind. For example, the Cultural Dark Age in Antiquity provides Missionaries in the Exploration Age an additional charge butimposes a Happiness penalty in all Settlements once you have founded a Religion in . This can be useful if you fail to construct any Wonders in the Antiquity Age, have a surplus of Happiness, and wish to pursue the Cultural Legacy Path.
The Scientific Dark Age is unlocked by completely failing to acquire a single Codex in the Antiquity Age. It provides a free Technology every five turns in Exploration, but cuts Science output in half. Depending on the circumstances or strategy, Science may not be a focal point, and you might actually unlock more technologies in the Exploration Age than you would otherwise with
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