Genmu Zero

Genmu Zero (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, Zero will keep spamming the attack, making it an impossible battle.

Other appearances
In Marvel vs. Capcom 3: Fate of Two Worlds, Zero can use Genmu Zero without being awakened, but only one powerful non-lethal wave is shot out instead, yelling "Ittouryoudan: Genmu Zero" (一刀両断・幻夢零) when using the attack (the kanji for zero can be read as rei, zero in on'yomi or just zero with a Japanese accent, thus there is no difference). It is also colored yellow instead of green. In Marvel vs. Capcom: Infinite, Zero may occasionally yell out "Ichigeki Hissatsu!" (一撃必殺!, Certain-kill in one blow!) before unleashing the wave.

Ittou Ryoudan is a term that refers to "cutting in two with a single stroke", and is the art of splitting someone in half in one stroke, thus proving the prowess of this one hit-killing attack. It is also one of the only few Hyper Combos that does one hit, until Marvel vs. Capcom: Infinite where it does a few extra hits as the wave impacts the opponent.

In Project X Zone, X and Zero's map attack has Zero using Genmu Zero and X using Hadouken at the same time. Here, Zero fires two green waves at once like the original version (along with a similar animation to the original sprite) and while also remaining grounded. Genmu Zero also appeared in Project X Zone 2.

This move also appears in Super Smash Bros. Ultimate. In that game, Zero can do this move in the ground and in the air.

Genmu Zero Kai
Genmu Zero Kai (幻夢零・改) is a different version of Genmu Zero used by Zero Nightmare in Mega Man X6. The Zero Nightmare releases one giant crescent wave from his saber, with varying speed. Depending on his level, Zero Nightmare can release two or more waves during the attack, one at a time. This move seems to be weaker than the original, mainly due to how incomplete the structure of the Zero Nightmare is in comparison to the original. Thus, this attack, like several of the Nightmare's other moves, is slow in startup and is quite delayed; the Genmu Zero Kai being no exception in comparison to his other projectiles, as they all vary in travel speed at a random rate, causing some unpredictability. Gameplay-wise, it is possible it was "revised" for more "practical" uses compared to the original, and while it is easier to avoid, it is still somewhat lethal.

Trivia

 * When using the Genmu Zero in Marvel vs. Capcom 3, Zero can be seen glowing with a faint red aura, referencing his awakened state.