Is a Dew. He comes in the appearance of a large black horse. This dew causes drought. Thirst can be used to inspire one to do all kinds of things.
Apaosa (Apa-urta) is a demon who brings drought and aridity. He rides on a black, bald horse. Eventually he was defeated by the god Tistrya. He is equal to the Indian evil spirit Vritra.
In a hymn of the Avesta (incorporated by Ferdowsi, with due acknowledgement, in the Shahnameh), Tishtrya is involved in a cosmic struggle against the drought-bringing demon Apaosha. According to the myth, in the form of a pure white horse the god did battle with the demon who, in contrast, had assumed the form of a terrifying black horse. Apaosa soon gained the upper hand over Tishtrya, who was weakened from the lack of sufficient prayers and sacrifices from humankind. The yazata proceeded to call upon the Creator Ahura Mazda, who himself then intervened by offering a sacrifice to the overwhelmed god. Infused with the power brought by this sacrifice, Tishtrya was able to overcome Apaosa, and his rains were able to flow to the parched fields and pastures unabated by drought. This story serves to underscore the importance of votive offerings and sacrifice in religious tradition.
Remember, the side of light and order is founded on pain and blood through sacrifice. This is another fine example.