The witch's cave is a small area that exists somewhere outside of Stardew Valley. At a certain point in the progression of the game, if the player is close enough friends with the wizard a cutscene will play upon the player entering the Bath House area north of the Carpenter's Shop. The wizard will be standing in front of the strange structure blocking the cave in the north-east corner of the area. He will ask the player to go to his ex-wife's house and retrieve his magic ink for him. He will then tell you to get a dark talisman, and to ask Krobus as he will know where to find it.
Once the player speaks to Krobus after experiencing the magic ink cutscene he will say he dropped a dark talisman in the cave at the north-west corner of the sewers and lower the force field for the player to pass. Once in the bug cave behind the sewers the player will have to fight through many green maggots and flies to reach a large chest at the rear of the cavern which contains the dark talisman.
Once obtaining the dark talisman the player can gain entry to the small cave behind the spa area. This cave holds a strange glyph in the center of the floor that will teleport the player to the witch's cave. In the witch's cave is a villager named Henchman who blocks the player from entering further as well as the Witch's Hut.