Mastermune (グランドリーム , Grandream?) is Serge's ultimate weapon in Chrono Cross. It has a near-perfect critical hit rate. It can be found after defeating Dario on the Forbidden Island. Mastermune deals extra damage to Demon, Dragon, Floating, Human, Marine, Matter, Soft, and Zombie enemy-types. Mastermune is a further nod to master swordsmith Masamune (hence Master + mune), widely regarded as the greatest Japanese swordsmith to have ever lived.
Transformed from the holy sword, Masamune, its inhabitants (Masa & Mune) awaken and dedicate themselves to be helping Serge. Doreen melds with them to form the weapon. When it is equipped and Serge uses his FlyingArrow tech, Masa & Mune can be seen behind him powering it up.