Jak 3: Part 23: Dark Jak Strikes Bak
3After going through the 2nd Sewer route in this game, you should end up in the Metal Head section of Haven City. This used to be the greener "farmland" area of the city before the Metal Head made it into a more toxic and sickly green" with acid rivers.
Ashelin will chime in and inform Jak that he'll need to take out at least 5 different caches of Dark Eco that the Metal Heads are using. The rivers will contain a lot of platforms and "rails" for you to stand and grind on in order to reach these caches. There are also weird plants that blow steam that jets you further ahead through this disgusting place.
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After hitting 5 Dark Eco caches, there will be a 6th one right at the giant purple wall. When Jak hits this, he's forced into Dark Jak, because taking down so much of these pods of Dark Eco inevitably saw their way into him, granting him a new Dark power.
From this point on, Dark Jak can use the Dark Strike, which lets him fire charged twin bolts of Dark Eco at his enemies and to destroy walls, just like the purple wall you need to destroy in front of you to complete the mission.
4:33 - HAHAHAHAHA!
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After you complete this mission, Samos gives you a new one. Although this area of Haven City has been dominated by Metal Heads, South Haven Forest is still mostly in one peace. Samos wants you to go into that autumn forest and purge the infested Dark Eco plants that are now scattered throughout the woods.