I am a God of Destruction

A young man dreamt of a strange entity offering him a gift, "You know of Izayoi Sakamaki? I gave him a gift but he ran away and didn't become my friend. Will you be my friend? I will give you the complete power that I gave him if you accept."
He managed to escape and wake up but...
He did not expect to awaken in another world, in another body!
And worst of all, a great old one wanted him dead.
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The story begins in Nasuverse but in the past not in the holy grail wars.