Quest 662

Walkthrough
Speak to Agatha in the Sword Bar once more who tasks you with defeating 3 dark creatures. To find the dark creatures however you'll need some help from the hooded man sitting on the upper floors of the bar. You should have seen him on your left as you came in.

Speak to the hooded man and acquire a sealed crystal which will draw out the dark creatures.

Next, head to the T. Coniware Plains and hunt down Marine Slimes, they're the slimes with seashells on their head. When encountering these slimes you should see an additional enemy, the dark marine slimes. You'll need to use the Demon Swordsman skill "Evil Flame Wave" the level 40 Demon Swordsman-exclusive class skill. You can find them around F-2 in the T. Coniware Plains.

Tip: If you need a fast way to get there, take the carriage outside T. gran Zedora Kingdom and select the fourth option. This will deposit you in front of the Sanmon barrier. from there head inside, and then to the West.

Once you've dealt with the 3 darkness slimes, return to the Sword Bar and report to Agatha.