The Ultra-Sphinx is a monster that had hounded Samson and Atlas since they had "borrowed" the Jewels of Atom-hotep (radioactive crown jewels from back in the first dynasty of Atom-Hotep, 80th century BC.
Before Superman's death, it said he had completed 12 super challenges, the stuff of legend. It said Superman had created life, escaped from the underverse, overthrew the tyrant sun (Solaris), and answered the unanswerable question.
In order for Samson and Atlas to get Superman to help them fight the Ultra-Sphinx, they had to trick him. They gave Lois the stolen Jewels of Atom-hotep as a gift. Angered, the Ultra-Sphinx grabbed Lois and said "Return what was stolen and pay the price." The Ultra-Sphinx had Lois in a condition of quantum uncertainty, where she was neither alive nor dead. It then asked Superman the question, if he was to answer correctly Lois would live, if he failed to answer correctly she would die. That was his judgement.
The question was "What happens when the unstoppable force meets the immovable object?" Superman responded "They surrender" which was accepted by the Ultra-Sphinx and Lois was allowed to live.
Ultra-Sphynx is a extra-dimensional being of extraordinary power. He is powerful enough to instill fear of his retribution in both Atlas and Sampson, who both possess immense physical powers (on par with Superman, when not Super-Charged by the Sun), as was depicted in "All-Star Superman." The UltraSphynx also possess a number of metaphysical superhuman abilities that are both mystical and cosmic in nature, the levels of which rival the abilities of the most powerful beings in the DC Universe.