Damsel in Distress

When the players are traversing the terrain over to the next settlement, they are approached by a child, clothes torn and dirt smeared by the brush of the forest. (Perception check may reveal that the child still seems well fed and not slimming at all from the days spent out here in the wilderness.) He will tell you that the temple knights have chained her up for being a witch and left him there with her after he refused to leave her side. 
If the party agrees to go with the child they will go an hours walk into the forest to an abandoned monastery. Outside a woman is seen with a wooden collar around her neck and chained to a post (the post is runed and evil can't touch it or the chain.) She and her "son" will testify that she had been left there to starve to death by the temple. If questioned, the woman will reluctantly admit she was caught in a promiscuous life style and found to be a whore is the same as being a witch to the firstborn. 
If the player removes the collar, she will cackle before thanking the player for being dumb and sprout wings to fly off after snapping her chain. 
If the player walks away, the boy will laugh and say to the woman he has fulfilled his bargain in spending a whole day in trying to free her. He thanks the players for not being as dumb as other mortals and drop toss them an enchanted dagger before supernaturally climbing up the monastery wall and out of sight. 
Dagger of Venom Source: Dungeon Master’s Guide
Weapon (dagger), rare
Requires Attunement: No
This ornate, curved, magical dagger grants you +1 to attack and damage rolls. You can use an action to coat the blade of this dagger in thick, black poison, which lasts for one minute or until you hit a creature with the dagger.
The creature must make a DC 15 Constitution saving throw or take 2d10 poison damage and become poisoned (disadvantage on attack rolls and ability checks) for one minute. Once used, the dagger can’t be coated in poison again until the next dawn.