Genmu Zero

Genmurei (幻夢零) is the attack Zero uses in his awakened state when X fights against him in Mega Man X5. If the battle against Awakened Zero continues for a long period (around 2 minutes), Awakened Zero will become invulnerable and launch two giant crescent waves from his Z-Saber repeatedly, which cover the entire area in front of him and can kill X instantly, making it impossible for X to win the battle after he starts using it, even if X manages to repeatedly use invincible attacks such as the Nova Strike just to stay alive.

In Marvel vs. Capcom 3: Fate of Two Worlds, Zero can use Genmurei (translated as Genmu Zero in the game menu) without being awakened, but only one powerful non-lethal wave is shot out instead, yelling "Ittou Ryoudan: Genmu Zero" (一刀両断・幻夢零) when using the attack.(The kanji for Zero can be read as Rei, zero in Mandarin, or just zero with a Japanese accent, thus there is no difference.) It is also colored yellow instead of green. Ittou Ryoudan is the art of splitting someone in half in one stroke, thus proving the prowess of this one hit-killing attack. 

Genmurei Kai
Genmurei Kai (幻夢零・改) is a different version of Genmurei used by Zero Nightmare in Mega Man X6. Nightmare Zero releases one giant crescent wave from his saber, with varying speed. Depending of his level, Zero Nightmare can release two or more waves during the attack, one at a time. Unlike the original, this move seems to be weaker, as it is possible that is was "revised" for more "pratical" uses unlike the original, though it is still somewhat lethal.

