Source Wall

"Look at them all, all those lives you could lead. Do it, John. Do it. Drink of The Bleed Mandrakk hid within the cube. Feel its power. Break through the Source Wall, for when you break through that wall, all those worlds that await will be yours."

- Eobard Thawne to John Diggle

The Source Wall is a barrier surrounding the multiverse.

John Diggle's Visions
Whilst viewing the lives that he could lead if he were to open the cube containing the Green Lantern Power Ring, John Diggle had a vision of the Source Wall, resulting in Eobard Thawne urging him to break through it, though he threw the cube away instead.

Entropy Wave
In the thirty-first century, the Legion of Super-Heroes travelled to the Source Wall to save a group of Dominators who believed they were being punished by their gods for leaving Dominion. Whilst doing so, the Promethean Giants began to crack, and the Source Wall was destroyed, resulting in an entropy wave being released across Earth-167.