The Black Rabite is the secret boss of the game, technically a megaboss however it
can be done on "Beginner" difficulty which completely trivializes the whole thing. I did it on playthrough 2 with my characters ranging from like 85-92 and nuked him in a few minutes.
Location and below info from the Trials of Mana wikia...
"Once a player has beaten the last story dungeon in Trials of Mana, they must then proceed to area that they visited just before. This will vary based on the main character that the player chose:"-
Angela/Duran must visit
Crystal Desert.
-
Charlotte/Kevin must visit
Mirage Palace.
-
Hawkeye/Riesz must visit
Night Cavern.
"Defeating the Black Rabite rewards the player with the Obsidian Power Chain Abiltity, which reduces the MP cost of any ability to 0. He can be re-challenged multiple times after that, but won't give any additional rewards, except for the rare drop of the Moogle Pin."Once you arrive at your location there will be a stone rabite statue at the entrance you interact with to be transported to the Black Rabite lair, i don't think you will need it but ill also link below a youtube video showcasing the fight