This page explains how to obtain Ending 8: “The Dark Fate” in Chrono Cross: The Radical Dreamers Edition.
How to Obtain Ending 8
To acquire this ominous ending, you must defeat the Time Devourer after rescuing Riddel, but before completing Chronopolis.
During this time, use the Time Egg at Opassa Beach and wallop the Time Devourer good. How many times have you defeated it now?!
For party members, you should be spoiled for choice, especially if you keep playing till you regain Serge. You won’t have the Mastermune, but a Spectra Swallow is perfectly fine.
This ending has potential, but is honestly pretty lame. Harle will arrive at Chronopolis and a fateful battle begins. Meanwhile, Dark Serge laughs maniacally as his goal is within grasp. Then it just ends. If you want a decent “bad ending”, check out the next one instead.
See Ending 8: The Dark Fate.