Dark Eco is a mysterious and sinister substance that appears in all Jak and Daxter games so far. In high amounts it can be deadly, and in small amounts can cause great damage on both sides. When harnessed correctly it can mean the end of any battle, but it is not a type of eco to be messed with.
Gol Acheron is the Sage of Dark Eco.
 Jak and Daxter: The Precursor Legacy
In Jak and Daxter: The Precursor Legacy, Dark Eco was left in random boxes, and when the player walked/drove into them, the player would get damaged. Dark Eco is also responsible for turning Daxter into an Ottsel; he accidentally falls into a pit of Dark Eco. An odd thing to note is that Daxter didn't get hurt by the Dark Eco, and doesn't seem to be affected by it throughout the game. This suggests that Daxter has somewhat of an immunity to Dark Eco. This is also what turned Gol and Maia into evil creatures.
 Jak 2 and 3
In Jak II: Renegade, Dark Eco plays a more vital role. As Jak has now become a Dark Eco experiment, he now has access to more abilities as Dark Jak. He no longer gets harmed by small amounts of eco (though he does get harmed in larger amounts) and he collects Dark Eco from fallen enemies to store in a dark meter. The same principal goes for Jak 3.
Dark Eco seems to have a strange attraction to Jak as whenever he comes close to the substance, it automatically comes to him. It is interesting to note that Dark Eco seems to be affected by the presence of Light Eco. This is suggested when Jak gets his first Light Jak ability. After he is imbued with the power of Light Eco, he says the following to Daxter;
"Dax! The Dark Eco...it feels...far away. I feel better!"
In Jak 3 Dark Eco is also available in Dark Eco vents, similar to Eco Vents from Jak and Daxter: The Precursor Legacy. These vents completely restore Jak's dark meter any time Jak walks into one.
 Jak X: Combat Racing
When the player has killed approximately 3 racers, the dark meter will be filled. When it has been filled, the player will have a dark purple glow around the car. This signifies that the car is now at its full potential and his faster and stronger. The player will have access to new and more powerful weapons, much more powerful rear defenses and an even faster turbo boost through the use of Blue Eco. Maxing out your dark meter is a devastating blow to opposing racers and can give you a great advantage to win races!
 Jak and Daxter: The Lost Frontier
In Jak and Daxter: The Lost Frontier, an evil group is tampering with Dark Eco using the Eco Core. Large amounts of Dark Eco can also be found in the field, like Jak and Daxter: The Precursor Legacy. Small drops of Dark Eco can be found and traded to Keira for more Eco powers. The Green Eco crystal, found at the beginning of the game, converts Dark Eco into colored Eco. It is also used to change Daxter into Dark Daxter. None of Jak's Dark Eco powers return; in the first cutscene of the game, Jak attempts to transform into Dark Jak, however he fails due to the shortage of eco.
The Aeropans are infected by large amounts of Dark Eco, and turn into Dark Eco Men. Tym - the Dark Eco sage - assisted in this project, later regretting it and attempting to take it back (he failed). Duke Skyheed uses Dark Eco to turn into an Eco giant - a huge beast with overwhelming power (even able to throw planes).
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