User blog comment:Legionwrex/Did the Leviathan DLC really add a new twist to the endings?/@comment-1388547-20120830204628

It might not be so bad as that. The Leviathans have been on the run for tens of millions of years, and have never been able to reclaim their former glory. They're good at hiding, and can take down individual Reaper capital ships if they're cornered, but they're certainly a lesser threat than the Reapers themselves. They also need their sphere artifacts to use their mind control abilities at a distance, and there is apparently a limited supply of those. I'd say that if the Reapers are still around (Control or Synthesis) the Leviathans have no chance at all of taking over, and with high EMS going into the Destroy ending, the modern races would probably be able to stop a Leviathan grab for power (though I'm guessing here). By contrast, a low-EMS Destroy, where the allied fleets are devastated by the Reapers or the Crucible's backfire, would give the Leviathans a chance to take over.