A large part of Chrono Trigger’s novelty lies in its multiple endings. On your first trip through the game you’re likely to get the ‘standard’ ending, but when you begin a New Game + you’ll have the equipment and the skills to bring down Lavos at virtually any point. Chances are, if you played through Chrono Trigger normally, after beating the final form of Lavos, you'll be treated to a good ending of a midnight stroll of the Millennial Fair, culminating in Crono's mother jumping into the final time portal and being stranded, forcing the characters to take the Epoch through time once more.

Part 26: Ending: The Oath

https://www.youtube.com/watch?v=O7On88Du3Qw
Finally, the endings get right back to being interesting again.
This one requires beating Lavos after recruiting Frog but before defeating Magus. Which means defeating Lavos despite going through all the effort to prepare Frog for that showdown he never gets.
https://mp4.mega64.net/ct/e09/000.mp4
So we start off with quite a racket being made, before we fade in to...
Peaceful Days
...Lucca's house. Everyone's just chilling besides a tense-looking Frog.
My apologies, Lucca...
It's not your fault. We're the one's [sic] who created weapons...
Phew...all right.
I think the localization of the endings were rushed a bit, that's two errors already.
Frog got all hyped up for his epic showdown against Magus, only for Crono and crew to fool him into helping them kill Lavos instead. He's not really in the mood to kick back and relax.
Instead of using JERK like I've been doing for these endings, I cheated my main file to keep the names intact.
I regret doing that now because this really is a JERK-style ending.
Interestingly enough, there's official art depicting this ending.
Confusing Melody
The credits now start popping up between scenes.
With the others not helping him, Frog decides to take on Magus and his castle alone.
That means having to face Slash...
...and Flea...
...and all one hundred monsters in Magus's Castle.
SILENCE
And Ozzie too but to be fair he doesn't really count.
Frog's Theme

Chrono Trigger Endings

Instead of foreboding silence, Frog's Theme blares for this bit where he triumphantly makes his way down to where Magus stands. Frog makes no effort of the forces within the castle, mere distractions before his inevitable duel against the wizard.
SILENCE
And things play out similarly as before...
Except this time, of course, Frog is alone.
Except for you and I, this world is populated by cretins...

Chrono Trigger Endings Walkthrough

At this point, Frog drops the act. You probably noticed by now, but the Olde Englishe accent does not appear once during this ending. Or, more likely, this ending's localization is rushed and Woolsey had no time to add it in.
Battle with Magus
Let's see if you know how to use it...
And so starts the epic battle between the great wizard and the noble knight.
https://mp4.mega64.net/ct/e09/043a.mp4
...which we only get to hear as the credits roll. Love the sound effects, at least!
Magus's Castle
Alas, there is only but one winner...