Once you have completed the second quest from Zack, An Agile Enemy, who can be found to the east of the camp, upon speaking to him again, you can take on another quest, called Getting rid of the Plague.
For this quest, you need to defeat five Plague Kahans. You've got two options here, head back to Velhora Waterfalls, to the west of Raven's Road Camp, or carry on with the story until you get to Vigga Marshes, to collect volatile liquid for Torus in the A Challenge in the Marshes quest. You'll easily find and kill five Kahans completing that quest by going through the area and solving the light puzzles there.
These creatures are the deadliest of the Plague monsters. They have the most health and a pouncing attack that will swiftly catch you off guard. Meaning you cannot continue to attack to kill them before they can attack back. Your best plan is to attack a few times, then evade, rinse and repeat until it's dead!
When five have been killed, once again return to Zack to learn an advanced sword technique, the ability to charge up your sword to deal extra damage on that swing.
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