The 13th Warrior should not be a Northman. So they recruit an Arab, Ahmed Ibn to be the 13th Warrior.
Why should the 13th Warrior not be a Northman?
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Because there was a prediction that they would fail if they didn't include someone not from the North.
Regardless of his feelings towards them, an old soothsayer whom the Vikings call the angel of death, cast the bones and warns the band that they will fail unless they are accompanied on their journey by a 13th warrior and one who is not from the North.
The soothsayer has, essentially, they believe, seen the future and can tell them that he will play an essential part in their struggle.