The Source Wall is a massive construct deep in space with large statues within it. When Darkseid launched an attack on Earth to avenge himself against Superman, Lex compelled Metron to help him defeat the evil god. Metron took him to the Source Wall, beyond which lay the one thing which could defeat Darkseid. Metron warned Luthor that only a twelfth-level intellect could survive passing through the wall but Luthor was unconcerned&managed to make it back with something that could stop Darkseid — the Anti-Life Equation.
The Source Wall, by Dan Schoening.