UPDATE: An English translation is now available courtesy of Sky! Check it out here: https://skytranslates.tumblr.com/post/180675133040/x5-if-translation-first-time-typesetting-so
According to various sources, the 10-page story takes place in the third Zero Space stage with X facing off against an "awakened" Zero. What ensues is garishly detailed fight that ends with Zero literally being blown in two and X dying by Irregular Zero's hands.
Now just a torso, Zero returns to normal and laments the death of his best friend. Zero musters his reaming strength and repairs X with the remains of his own body. A foreboding, yet familiar voice then beckons to Zero – Sigma.
Sigma approaches and confronts... this fella:
|X? Or "Xero"?|
Although it appears to be a literal combination of X and Zero, Sigma allegedly identifies this figure as "X". We're led to assume Zero successfully repaired X at the cost of his very own life. (what was left of it, at least) Iwamoto, my man, you've really outdone yourself here.
So right about now you're probably wondering where you can see more of this fascinating tale. At present, you can only find it in the 25th anniversary Memorial box set. I'd like to say let's hope for UDON to swing in and bring this one stateside but, alas, Iwamoto's work remains stuck in Japan.
Many thanks to CapKobun, GARUME, and LadyZure17 for details and images!