HRUNTING/YGGDRASIL Mark II [D] Geyrion
From Halopedia, the Halo wiki
The Mark II [D] is an extremely large vehicle, towering over humans - even when crouched. Its overall form bears a large outward resemblance to the later Mark IX Mantis pressed into service as a more standard vehicle. Like the Mantis, it is armed with a multi-barreled cannon on its right boom and an array of missile launchers on the other.
The Geyrion is a part of the Mark II project line of exoskeletons, developed by the Materials Group as part of joint projects HRUNTING/YGGDRASIL, illustrating the limits of human technology. The Geyrion was used to prove out the basic idea of large bipedal walkers, and was successful in proving capable in early trials conducted in microgravity environments. Unfortunately, the Geyrion suffered in more regular Earth-like terrestrial environments, and its limited mobility proved to be a massive hinderance in all but the most specialised deployments following the end of the Human-Covenant War. A next-generation successor to the Mark II [D] was eventually developed by Hannibal Weapon Systems and the Materials Group, known as the Mark II [J] and commonly nicknamed the "Colossus".
A Mark II [D] aboard the freighter CAA Heavy Burden serves as the centerpiece of the Halo 4 multiplayer map Adrift. In an interview Frank O'Connor called the mech a "Cyclops Mark II", referring to the Cyclops from Halo Wars. However, Halo 4: The Essential Visual Guide makes no mention of this name. Since "Cyclops" is merely a colloquialism for the HRUNTING Mark III [B], itself derived from the third-generation exoskeleton that predated MJOLNIR, the "Cyclops Mark II" moniker was probably used only from a game-design perspective rather than having any canonical significance.
In Greek mythology, Geryon is a large giant, and presumably the namesake of the Geyrion vehicle. It is unknown as to whether the Geyrion's name in the Halo Encyclopedia (2022 edition) is an intentional misspelling or a typo.
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