Mysteltein is an AAA-Class Red Sidereal piercing weapon that can also used to slice through objects like a double-saber. Not only can it be thrown at extremely high velocity, but EX-Type Zero Hyperion demonstrates that its direction can be controlled while in flight, leading to utter devastation of multiple targets very rapidly. It was given to him by Bishop.
It has been called the "cursed branch" and the "cursed bough" due to having some kind of mutagenic effect on pierced targets. This causes rapid disintegration of non-biological objects like ships, wounds that do not heal easily and contamination of surrounding land. It seems similar to the corruption that happens around Queen Nests and fallen Blue Flower debris.
After the defeat of Hyperion it was was recovered and studied in order to use its power for the spears wielded by the Raven Units. Speaking about Zion being subdued by these spears, "There's no point. That's our ace in the hole to substitute the AB Element. She won't be able to resist. Those spears are just replicas... ...that only resonate the original. And the curse that enables the resonance comes from... Mysteltein. The cursed piece of wood from the enemy." Robert Nelson, (AE2 - Ch30)
"It was a terrible curse... 30 years have passed, but no one can inhabit on the earth that was struck with that bough." Tinie, (AE2 - Ch17)"That thing's holding... The nightmare of planet Arle." Random solider, speaking about Hyperion. (AE2 - Ch56) When Hyperion Repair later appears he summons Mysteltein from a great distance, totally destroying its field vault for the Raven spears as it instantly responds.