Kill (or save) a deadly beast responsible for razing a merchant camp.
The Witcher 3: Blood and Wine - Mutal of Beauclair's Wild Kingdom
Directly North and a little bit to the East of Coronata Vineyard (as discovered in the Wine Wars quest line) is a trading post. Head here to find a slaugher; the entire ost has been laid waste, the work of a beast. You'll want to be around level 46 to tackle this quest, as there's a fairly challenging boss at the end of it.
You can either stumble upon this camp by exploring or following the directions given above or receive this quest from one of a few notice boards around Toussaint, including the one at the Cockatrice Inn, the first inn you visit as part of Blood & Wine's main story.
This is a fairly standard monster hunt as found in the main thrust of The Witcher 3's content. As such, you'll want to use your Witcher senses around the camp to investigate it; this will trigger the appearance of an NPC who doesn't want you to kill the beast in question, and explains all about the beast to you in detail.
Use your Witcher senses to follow the scent of the beast after you find its scent on two items in the merchant camp. Keep on following - this is all very much scripted and impossible to screw up. Things become more interesting when you find a group of warriors, the Reavers, at the lair of the beast.
You're now faced with a choice: side with the Reavers and kill the beast, a Basilisk, or side with the NPC who's followed you since the camp and spare its life, since it's supposedly one-of-a-kind.
Here's the upshot: If you kill the beast you'll get 200 experience, 400 coin, and you can loot the beast for a sword, some mutagen and the creature's trophy. If you choose not to kill it you'll get a little more cash in the form of crowns and around the same experience, and of course a different narrative outcome.
If you carefully read the quest notes you'll see that the story of the Basilisk attacking the camp isn't quite as black-and-white as some would like to claim - so I'll leave you to decide exactly what to do on your own.
If you choose to fight and kill it be aware that it's a tough fight, and I find the better option was to have the Reavers support you with crossbow fire. Draconid oil for your sword will prove very useful indeed. Igni and bombs also work well, and though you have the assistance of the Reavers can be useful to bring it crashing to the ground if it's airborne. Don't go too crazy when it gets knocked down - remember it can use its wings to block your attacks and unleash a nasty counter. Good luck!