" Angra Mainyu  "

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Zoroastrianism, regarded as mankind's oldest good and evil dualism doctrine. A Servant crowned with the name of the devil king transmitted in said Zoroastrianism.
Only the summoner can judge whatever he is the true Angra Mainyu or not.
An Heroic Spirit specialized in killing humans, but he can't do anything other than that. In the Holy Grail War, a place of battle for super-humans, he is of no use whatsoever.
The weakest Servant, in both name and reality.