Here's a short story:

Zoroastrianism, or more natively Mazdayasna, is one of the world's oldest extant religions, "combining a cosmogonic dualism and eschatological monotheism in a manner unique [...] among the major religions of the world". Ascribed to the teachings of the Iranian prophet Zoroaster (or Zarathustra), it exalts a deity of wisdom, Ahura Mazda ("Wise Lord"), as its Supreme Being. Major features of Zoroastrianism, such as messianism, heaven and hell, and free will have, some believe, influenced other religious systems, including Second Temple Judaism, Gnosticism, Christianity, and Islam. 

With possible roots dating back to the second millennium BCE, Zoroastrianism enters recorded history in the 5th-century BCE, and along with a Mithraic Median prototype and a Zurvanist Sassanid successor it served as the state religion of the pre-Islamic Iranian empires from around 600 BCE to 650 CE. Zoroastrianism was suppressed from the 7th century onwards following the Muslim conquest of Persia of 633–654. Recent estimates place the current number of Zoroastrians at around 190000, with most living in India and in Iran and their number is declining. Besides the Zoroastrian diaspora, the older Mithraic faith Yazdânism is still practised amongst Kurds. 

The most important texts of the religion are those of the Avesta, which includes the writings of Zoroaster known as the Gathas, enigmatic poems that define the religion's precepts, and the Yasna, the scripture. The full name by which Zoroaster addressed the deity is: Ahura, The Lord Creator, and Mazda, Supremely Wise. The religious philosophy of Zoroaster divided the early Iranian gods of Proto-Indo-Iranian tradition, but focused on responsibility, and did not create a devil per-se. Zoroaster proclaimed that there is only one God, the singularly creative and sustaining force of the Universe, and that human beings are given a right of choice, and because of cause and effect are also responsible for the consequences of their choices. The contesting force to Ahura Mazda was called Angra Mainyu, or angry spirit. Post-Zoroastrian scripture introduced the concept of Ahriman, the Devil, which was effectively a personification of Angra Mainyu. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What does Zoroastrianism combine?
Guess: It combines a cosmogonic dualism and eschatological monotheism in a manner unique [...] among the major religions of the world"

Question: What are the most important texts of the religion called?
Guess: Avesta

Question: who's writings does it include?
Guess: the writings of Zoroaster known as the Gathas

Question: When do it's roots possibly date to?
Guess: the second millennium BCE

Question: to which Iranian prophet does it's teachings ascribe to?
Guess: Zoroaster (or Zarathustra)

Question: When did it serve as the state religion of the pre-Islamic Iranian empires?
Guess: from around 600 BCE to 650 CE

Question: When was it suppressed?
Guess: the 7th century onwards

Question: What diety does it exalt?
Guess: Ahura Mazda ("Wise Lord")

Question: What is the current number of Zoroastrians estamated?
Guess: around 190000

Question: How many gods does Zoroastrianism beleive in?
Guess: one

Now answer the following question in the same format.

Question: What is the contesting force to Ahura Mazda called?

Angra Mainyu