This side quest starts in Lurth village (M9,20) in Velen. There you will find feasting bandits, talk with them. You can pick peasant's Yontek side or decide not to interfere - if you remain neutral, the quest won't start.
Get rid of the bandits (level 4) and talk with the peasant. Go to the marked house and examine any pig, then resume the conversation. You will receive a key. Now go to the cave not far to the east. Before the entrance you will find three more bandits. Another few will be inside. Use the key on the passage at the end of the corridor and behind the portal you will reach another corridor.
You will see some prints on the ground, and after a while you will notice a pig - the transformed herbalist from the village. Talk with him, then walk to the altar at the wall and examine it. Then talk again with the pig. After getting back to the village you will face 3 wolves. Once you get rid of them, talk with Yontek.
You must gather all the pigs in the village (there are 9 of them). Luckily, all you need to do is enter each of the three huts and use Axii sign on any pig for all of them to follow you. Once you reach the windmill, wait for Yontek and together go along the path. A small group of nekkers will attack you, after defeating them you will be transported to the cave where the curse will be lifted. Now you can decide upon Yontek fate - you can scare the peasants or let them do whatever they want with him. If you won't interfere, you will receive 100 crowns. Otherwise, you will get nothing.