Legend of the Void
Information
The Legend of the Void quest can be started during Chapter 13, after gaining access to Sashila Village. It rewards the player with 
12,500, an Absolite and +1 Karma.
Guide
Head to house to the west of Sashila Village, right beside the study library. Speak to Marten, who will request that the player speaks to a man named Hyu'ro, who is located inside the lowest house on the left side. Ask him some questions and then report back to Marten. After this, Marten will request that you enter the Voidal Chasm and look for something Hyu'ro lost on the second level. Enter the Voidal Chasm, head to the second floor and look for a spinning object. It should be fairly easy to locate. Once you've found it, return to Marten, who will request that you meet him at Hyu'ro's house after you've spoken to him.
Once you've met Marten at Hyu'ro's house, meet him at this library and speak to him, then use the Timesplicer Crystal in Alexandra's room to return to Hiyoshi City in the past. Speak to the truck driver and ask about Gregorian Lake, having them take you to it. Once you're at the lake, surf north and enter the house to receive the Lake Postcard, then return to Hyu'ro and show him the postcard. After this he will go to the Voidal Chasm again. Follow him at Marten's request, where you will find Hyu'ro being attacked by an Absol. Catch it to continue.
Speak to Hyu'ro after catching the Absol. You have the choice of allowing him to go deeper into the void, or having him return to Sashila. These options don't matter as of right now, but telling him to go deeper will give the player some lore. Return to Marten after deciding Hyu'ro's fate to complete the quest.