Mephisto's idea was actually very good. Although Trag'Oul was only an immature world-soul titan, he was still living on Sanctuary after all. Back then, neither the High Heavens nor the Burning Hells had discovered his existence, showing the secrecy of the spatial crevice he lived in. If not for the destruction of the Worldstone later and him taking the initiative to appear and hide on Sanctuary once, perhaps no one would know that he existed. And Mephisto had paid an unknown price and promised to reach an agreement with Trag'Oul to hide the six demon kings in Trag'Oul's spatial crevice. Under normal circumstances, they might really have been able to hide.
Unfortunately, Roy and Lilith had seen through his plan. In the duel between the seers, Lilith was still superior. Moreover, after she recovered her Deadly Sin-level power, she easily found the spatial crevice where Trag'Oul was hiding and found them all.
