Location: Peach Blossom Forest
Prerequisite: Complete Lou Fuzhou's Ending and Siege of Mount Windless
Petitioner: Leng Wuqing
Reward: Suzhi Saber Advanced Blueprint, Five Balances Sutra, Gold-Woven Belt Advanced Blueprint, The Six Knacks

In order to trigger this quest you need to complete Leng Fuqing's storyline up to Prelude to Counterattack, at which point he'll disappear. You then need to complete Lou Fuzhou's Ending, which recruits Ye Fei into the party, and Siege of Mount Windless, a main story quest. Phew.

Head to Peach Blossom Forest once you've done all that. This is where you met Wuqing previously, and he'll be on the same small plateau as before, east of the entrance. He has unfinished business with Liushan School, and wants to continue the investigation. He'll disappear after the conversation, and you'll need to follow him to Gusu City. (If you have Shangguan Hong and Sima Ling in the party they will comment on all this in a short aside.)

Head west of Peach Blossom Forest until you see a northbound bridge, cross the bridge, and then head east to find Gusu City. Visit the inn and you'll find Wuqing at a table on your right. He'll tell you to rest up for a few days and wait for your prey, Seaperil Dragon, to show up in Gusu. Prep your party, then speak to the innkeeper to skip forward in time. A battle's waiting on the other side.

Seaperil Dragon

Time to root out some corruption. Seaperil Dragon is a powerful opponent, far more powerful than Yi is likely to be at this point in the game, and you'll only have two people helping. (Fortunately, they're both quite strong.) Seaperil Dragon uses the following attacks:
  • Three-square saber attacks
  • A six-square saber attack that can inflict Poison
  • Bloodstained Devil's Saber, a diamond-shaped AOE that can drain health
None of your normal characters besides Yi will participate in this fight, but you do have the assistance of Leng Wuqing and Jue Yuxin. Wuqing is likely to be your greatest damage-dealer, so keep him back from Seaperil Dragon, using your other two characters as shields. Keep Seaperil Dragon distracted long enough and Wuqing will, eventually, pick through his HP and bring the villain down.

Defeating Seaperil Dragon ends this short quest, earning you two powerful Cultivation Methods in the process. Leng Wuqing will jump into your party, permanently this time, and he'll make a powerful ally indeed.