...I never played Chrono Cross, but the tie-ins found in the additional cutscenes are pretty obvious, and weird to me: https://www.youtube.com/watch?v=CkWK-NEtmHk
Particularly at the end.. spoilers, of course.
Chrono Cross spoilers:
During the fall of the Kingdom of Guardia in the year 1005, someone steals the Masamune. Masa and Mune have been sleeping for centuries now (Probably since Frog handed the sword over to Guardia). The sword was probably used for bloodshed, and without the two spirits inside to purity the sword, it became full of evil and hatred, changing its form and corrupting everyone who touched it.
It is unknown how the sword found its way into El Nido, but it is possible that it was in the possession of Porre, and it was left there in the main island by them.
It is unknown how the sword found its way into El Nido, but it is possible that it was in the possession of Porre, and it was left there in the main island by them.