Location: Tancheng City
Prerequisite: Begin Soulwrecking Song - Overture
Petitioner: None
Reward: None
This side quest is part of a series of side quests that focus on the Beggars' Sect, headquartered in Tancheng City. In order to trigger these quests you need to travel to Tancheng City after completing Siege of Mount Windless, then speak to the Beggars' Sect in the north of the city and agree to help them with their problems.
When you first arrive in Tancheng City during the whole Beggars' Sect fiasco you'll see an old man speaking to himself. You'll spot him again later, after speaking to the sect's leadership, which triggers Troupe History. Follow him west to Tancheng City's inn and you'll find an informative Waiter. He'll point you towards Nanyu Village, to the west of Tancheng City.
Travel over to Nanyu City and check the west side of the settlement. Here you'll find Uncle Yu, the old man, though he's reluctant to discuss his affairs with Yi. Eventually he'll agree to discuss what he knows about Soulwrecking Bird, enemy of the Beggars' Sect - if Yi can bring him some Twin Roseleaves. They're rare flowers found only in Bramble Shade Forest.
Travel back north to Wudang Sect, then head southwest to find Bramble Shade Forest. It's on the border between the Great Plains and the green, forested areas that you (likely) haven't visited. Walk a little ways south of the entrance, then take a left at your first opportunity to find the Twin Roseleaves. Someone else will show up to interrupt the harvesting, and Yi will charitably allow them to leave with the flowers.
Trek back to Uncle Yu in Nanyu City. He'll give Yi a new task: Fetch a Wedding Dress from Purple Bamboo Forest. You'll find the forest on the south end of Tianming Village, which itself is south of Tancheng City. Cross the bridge in the south of Tianming Village, then take a left to find the path to the forest.
There are two general paths through Purple Bambo Forest. Stick to the lower path, heading east until you cross a bridge made of bamboo. There's a clearing on the other side of the bridge, and entering it will trigger a cut scene where you find the Wedding Dress. You'll also have to defeat two Xuanhou Cult Strongmen, though they're normal enemies for this area.
Head back to Nanyu City once more and speak to Uncle Yu. A surprise third party will appear, at least temporarily, and after they leave Uncle Yu will give Yi an Unsigned Letter. Read the letter via the menu - it appears as an item - then speak to Uncle Yu for some clarification. This will end the quest, and you'll receive a Letter from Uncle Yu.
(And, yes, this is all quite important, if you want to recruit two new party members. Make sure you do this quest.)