The writing's on the wall
A biblical idiom that refers to something being revealed. In this case, it’s his friendship with Em and Jay-Z.
The phrase originates in the book of Daniel, where supernatural writing predicts the demise of the Babylonian Empire.
Something being revealed as destiny or fate.
It’s not his friendship with Em and Jay what he means is that the writing is on the wall or it’s clear to see he is destined for greatness since he is with Eminem and Jay-Z. So he is sitting between Eminem and Jay-Z at a ball game is the writing on the wall that he is destined or gonna be between Eminem and Jay-Z in the rap game. He is destined to be up there with the Greatest Of All Time.
In the original Slaughterhouse track Royce has the lines
“Now I’m spittin sittin between Shady and Jay (ohh!)” meaning he is spitting on the level with the Greatests Of All Time
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