THE SITRA AHRA and the powers of impurity feature in early folk tales, as well as in stories of kabbalists. It is therefore only to be expected that they would be present in hasidic stories, which drew upon these sources. The usual habitat of these evil powers, which are termed letsim (literally, ‘mockers’), ḥitsonim (‘external ones’), or kelipot (from kabbalistic terminology), was far from any settled human domain: the wilderness, the forest, abandoned buildings, and the like. They could, however, make their presence felt anywhere, and confront a person at any time. If a person follows the paths of Judaism and fears the Lord, he is assured that the powers of holiness will aid him; he will prevail over the dark forces, which will be sent away to their natural abode. For example, the Ba'al Shem Tov drove out demons who sought to attack the shopkeeper's carriage in which he was travelling with a merchant through the forest, and by doing so intensified the shopkeeper's belief in him.

Letsim or ḥitsonim, collectively known also as mezikin (agents of destruction), were liable to be generated by sins or bad thoughts: we learn, for example, that two letsim, a male and a female, were created from the sinful thoughts of an assistant cantor (‘a poet [i.e. singer] who was called a bass’) and from the women who were in the synagogue. The mezikin manifested themselves in the women's section of the synagogue; the women feared to pray in their presence, and were compelled to leave.

In other instances, these entities arise as the result of sorcery. For example, R. Moses of Kosov's daughter-in-law, experiencing a difficult childbirth, turned for relief to sorcery. This resulted in a lets coming to R. Moses’ home, forcing him to leave the house. The mokhiaḥof Polonnoye, Aryeh Leib Geliner, attempted to live there afterwards, but he too was compelled to abandon it by the lets, who threw wooden beams from the upper storey, and poured dirt on the books of the mokhiaḥ's companions when they fell asleep while studying. After the mokhiaḥ's departure the house fell into disuse, and various people attempted to remove wooden beams and benches from the deserted ruin; but the lets knocked on their windows at night and demanded that they return what they had stolen.