Limit Breaks are a feature of the enchant changer mod. When the green meter on certain weapons is full and the user has very low HP, they can be activated by shift+right clicking. The Limit Guage fills very quickly by attacking enemies. Every weapon with a limit break has a different break, and some are inherantly better than others presumably taking longer to charge,
Sonic Break Edit
Sonic Break is the Limit Break of the Buster Sword, named after Cloud's soul break from Final Fantasy Record Keeper and Dissidia Final Fantasy. It adds the haste buff for roughly 10 seconds.
Cross-slash Is the limit break of the First Sword.
Oblivion is the Limit Break of the Masamune Replica.
Octoslash is the Limit Break of the Masamune, Sephiroth's exclusive weapon. It floods the screen with just about every buff for roughly 10 seconds.