Ryuenjin

Ryuenjin, or Ryuuenjin (龍炎刃? lit. "Dragon Flame Blade"), is the special technique that Zero learns after defeating Magma Dragoon in Mega Man X4. By pressing the S.M. Button and UP on the D-pad while Zero is on the ground, he will jump up diagonally with a fiery Z-Sabre blade. This technique is quite powerful and is capable of striking multiple hits on a target, with the possibility of destroying them. Because of its nature, Ryuenjin can be used to burn the wooden pieces in Web Spider's stage.
In Tatsunoko vs. Capcom and Marvel vs. Capcom 3, this is Zero's main anti-air "Shouryuuken" style special (F, D, DF + Atk). The stronger the version, the more hits and height are put into the move. He is also able to use it as his primary assist attack in TvC (though in MVC3 he is able to have Hadangeki and Shippuuga as assists).
In the former, Zero can perform this in midair and a flameless version of the attack appears as his Variable Counter (a possible noding the original "Rising" skill as well as some of Zero's other rising slash variations that are purely physical with no elements). It is often the main air combo finisher to most of his air combos and is a bit subpar for an assist (though it can help with some combos if used right).
In the latter game, Zero can no longer perform this in midair due to the inclusion of Raikousen (which has the same command as Ryuuenjin but only in midair).
Ryuenjin causes a weakness reaction against Cyber Peacock, Frost Walrus and Reaper Sigma's first form. It is also the technique which causes the most damage to Iris and Sigma's cannon form.
Ryuenjin is also used by Omega and Model OX in Mega Man Zero 3 and Mega Man ZX.