The Dueling Families - Baldur's Gate 2 Side Quest
The Dueling Families of Trademeet is a quest that only becomes available after you're named the Heroes of Trademeet. You will be named the Heroes of Trademeet once you complete both the Animal Troubles in Trademeet as well as Dealing with the Trademeet Genies quests. Keep in mind that poisoning the Druids during the Animal Troubles quest will result in you not being named the Heroes of Trademeet; you must resolve the quest in the peaceful and less destructive way.
Once both of these quests are complete you'll get a ceremony where you're named the Heroes of Trademeet and during this ceremony you'll be approached by Lady Lilith Lurraxol and Lord Skarmaen Alibakkar. Both of these families will ask you to meet them back at their house for a job.
Each family will task you with the same thing, going into the crypt in Trademeet and looting the Mantle of Waukeen then returning it to them. I recommend you side with Skarmaen because this gives us a few more options for completing this quest. He has the Elven Chain Mail item which is by far the most important item in this entire quest and something we want to get while doing the quest.
The crypt that we need to visit for this quest is in the northeastern portion of the Trademeet map. It's the only building near the graveyard so it shouldn't be too difficult to find. You'll be approached by High Merchant Coprith when you walk up to the crypt and open the door who will ask you to give the Mantle of Waukeen to him instead. This gives you 2 different options for completing the quest.
You can give the mantle to Coprith which will cause the family you did not agree to help to permanently leave the game. If you took my earlier advice and agreed to help Skarmaen then he'll still be in the game and Lilith will have left. This means you can kill Skarmaen after you turn in the quest for his Elven Chain Mail.
Our other method for resolving this quest is going to be handing the Mantle of Waukeen to either of the 2 families. If you give the mantle to either family it will always end in conflict with both heads of the family attacking you (and each other). This is typically the outcome that I always go for since all I care about is the Elven Chain Mail.
Below you'll find the 2 different outcomes for this quest and more or less what to expect for each of them in an easier to read format.
Give the Mantle to Skarmaen or Lilith: When you give the mantle to one family it'll result in the other family storming into the building with some bodyguards and combat ensuing. It doesn't matter if you give it to Lilith or Skarmaen you'll also end up fighting both families. Thankfully this method will get us the Elven Chain Mail item along with whatever else the enemies drop.
Give the Mantle to Coprith: The only benefit to giving the mantle to Coprith is you will get +1 Reputation and blood won't be spilled. Keep in mind that when you give Coprith the mantle whichever family head you did not speak to at the start of the quest will leave the game forever.