Ryuenjin

"Ryuenjin: Up + S.M. button on the ground. A powerful attack that burns enemies with flames. Some objects may be burned with this technique."

- In-game text

Ryuenjin, or Ryūenjin (『龍炎刃』), also known as "Dragon Flame Blade", is the special technique that Zero learns after defeating Magma Dragoon in Mega Man X4 