This is in the Varhold's lot DLC, and will require passing some skill checks to get the best ending. When building your 4-person party, make sure you have good perception, persuasion, athletics, Lore Nature. and knowledge Arcana among your members. Also, Save after every success, and reload after any failure.
After returning from dealing with the Centuars in the Cyclops ruins, you'll start "Of Cows and Other Matters of Political Importance." Go to the inn, and talk to the Zombie Jester. Before leaving Varnhold, tell Meager Varn about the rumor the jester tells you. He'll stop by the Blackstones Ford Inn on your way.
Talk to the visiting merchant, and the owners.
Inspect three things at the caravan, outside (southeast) of the inn.
Inspect the dog and the book he's been reading East of the inn.
Go inside the inn. There's a perception check to notice a hidden door on the left. Inspect the things inside it. There's a Lore: nature check directly ahead of you. Go up the stairs. There's a few things to inspect upstairs, including a knowledge arcana check on a wand. Leave the inn.
Go Northwest to a cave. Pass the athletics check to get up there. Inside are traps and swarms, so be prepared. At the end, inspect the bodies.
Return to Varn. Save. In the ensuing conversation, urge waiting and peace at every step.