Mordru used to be THE big threat in the 31st century. The reasons for his drop in power are part of the gigantic cluster**** that is the LSH continuity.
I'm posting the Wiki article, but I don't know how accurate it is.
Val is the only person on here who I think knows the LSH enough to answer questions.
In the V4 Legion, it was revealed that originally Mordru was fated to rule the universe for a thousand years. Seeing this timeline, the Time Trapper caused the Legion to be founded in order to block Mordru's rise to ultimate power. However, when the Legion defeated Darkseid, the Time Trapper realized they had become more powerful than he expected, and were a threat to him. After Valor destroyed the Time Trapper, this timeline did indeed come to pass (for a single issue) before the previously minor villain Glorith cast a suicidal spell that inserted herself into history to take the Time Trapper's place as a time manipulator who would (indirectly) create the legion to block Mordru.
In this new timeline, Glorith was orchestrating a battle between the Legion and Mordru in which they would be evenly matched, destroy each other, and let her pick up the pieces as the only remaining power in the universe. Ultra Boy however figured out her plan, and turned the tables on her, visiting Mordru in disguise and provoking him into attacking Glorith before she was ready (but still more powerful than he expected). The result was a stalemate which cost both Mordru and Glorith the bulk of their power. In this retcon, this explains the sudden drop in Mordru's meteoric rise to power referenced above. He was still powerful, but had to content himself with ruling what he had conquered thus far.
