The Darkness.

For a moment there, they lost their identity. Legion-5 for sure forgot why he was in the void and what he was doing in the middle of a lightning storm. He was confused but only for an instant. The other clones helped him with his identity and purpose. Then Legion-7 restored his fragmented consciousness.

Legion-5 jested, "I think the void universe wants to kill us."

He is joking to mask the pain. Despite reaching what he thought was the limit of pain, they still felt more pain because of the last attack. 

Pain is the psychological response to damage to existence. It helps to determine a phenomenon that can threaten one's existence. That violet lightning is a threat to their existence hence the pain. 

He might have joked about it but Legion is certain that the void universe wants to kill them. It has not been up to a minute but they have already been pushed to the closest point of death that they have ever reached since they reincarnated.