Hunting A Witch - Velen
When you arrive in the town of Midcopse you'll be tasked with asking the residents about the Witch, there are technically three people you can ask but only two of them are easy marks. By the towns exit you'll find two women gossiping that you can eavesdrop on to learn about the Witch - or on the western end of town speak with old the man sitting next to a tree (pictured above).
Now that you've acquired the Witch's location you'll want to head over to her hut which is now marked on your map. When you arrive there will be a group of villagers begging her for favors, she seems pretty annoyed but helps them anyway before going back inside. The peasants trash talk her after she leaves, being the terrible people they are.
Head inside of the building and inspect the Skull next to the pentagram on the ground to create a portal (pictured below). Head inside of that portal to meet the Witch one on one.
The Witch's name is Keira Metz and she is one of the characters that you're able to romance as Geralt in the game. Once you speak to her you will complete this quest and unlock another one called Wandering in the Dark. This too is a story mission.