The Lump was a misshapen creature that lived deep within the planet Apokolips, in Granny Goodness's orphanage in a section called Section Zero. The Lump would bring his victims into his own mind, in a realm called The Arena of the Gods.
It was here he fought Mr. Miracle. The Lump was able to shift his body into different weapons, and fought Mr. Miracle to a standstill. However, when Mr. Miracle shot a energy blast at the ground to turn it to glass, he showed the Lump its true form. The Lump, horrified, retreated into the deepest depths of its mind.
Many years later, during the Final Crisis, Simyan and Mokkari used the Lump to help create an army of Batman Clones. Disguised as Alfred Pennyworth, it went about taking Batman through his memories, trying to imbue the clones with the determination and skill of Batman. However, he slipped when he had Alfred mention his time as the Outsider- something Batman had been careful to never mention to Alfred. This lead to a mental battle between the two, with the Lump forcing Batman to endure the worst mental anguish he had been through.
The raw emotional energy caused by this began killing the Batman Clones. To stop the mental link, Simyan and Mokkari shot the Lump to death with "soulkilling shells". Batman swore if the Lump helped him, he would find the Lump's killers,
The gun that was used to kill the Lump is the gun Batman used to kill Darkseid shortly after.