Gungnir is a weapon of mass destruction developed to destroy the Yggdrasil cores. Knowing that the cores would become monsters after the Yggdrasil Trees had absorbed the world's pollution, Gungnir was developed by the Yggdrasil Project scientists as a means of stopping them. But the consequences of using of the weapon are so great that initial use of the weapons were deemed impractical and were set as only a last resort. While seven Yggdrasil Trees were planted around the world, only two Gungnir unit stations are known to exist.
So far, the Gungnir units have been mentioned only in the story mode of Etrian Odyssey Untold: The Millennium Girl.
Etria's Gungnir is at first mentioned by M.I.K.E. after restoring Frederica's memories as being located within the ruins of Gladsheim, being said to be the only way to stop the Yggdrasil Core. Through the game, the Highlander's party explore Gladsheim's ruins seeking to release the locks that seal the weapon, unknowingly of its true nature.
Only after reaching the lock in Gladsheim AREA IV does Frederica explain the impact of using the weapon; while it will destroy the core, she had M.I.K.E. calculate the collateral damage the weapon would bring with its use, and shows that it would not only destroy the Yggdrasil Core but also much of the surrounding area, including Etria and its inhabitants and making the area uninhabitable. Upon finding this, the party objects against its use, deciding to find another way to bring an end to the Core.
M.I.K.E. however rebels against their decision, as activating the Gungnir was his sole purpose, and without it he will be rendered useless. M.I.K.E. attempts to take Ricky back by force, in order to brainwash her to go along the original plan, ultimately failing and being left alone in Gladsheim.
His crisis getting worse, M.I.K.E decides to activate Gungnir by himself, unlocking the final lock and activating its countdown, even though the weapon wouldn't have enough power to reach the core but still having the same devastating effects on the area above. However, the party ultimately defeats M.I.K.E., shutting him down and stopping the countdown, ultimately putting an end to the weapon's threat.
Visil, the Chiftain of Radha Hall and a member of the Yggdrasil Project, at one point in the story forbade Frederica's party from exploring Gladsheim any further, knowing about M.I.K.E. and Gungnir and seeking to protect both Etria and it's Yggdrasil Core (due to it partially brainwashing him).
A second Gungnir unit is mentioned in the game, being positioned in the vicinity of a mining city called Gotham, a town also settled in the reach of a Yggdrasil Tree. When the city's Yggdrasil went berserk and the Core awakened, this unit was activated, completely destroying the monster and the entire city. Simon Yorke and Arthur Charles, who hailed from the city, were two of the few survivors of the catastrophe.
There is no official image of a Gungnir unit that has been mentioned or shown in any of the Etrian Odyssey games. It has only been vaguely described by Frederica Irving and M.I.K.E. as a weapon capable of mass destruction. In fact, activating Gungnir was their mission in the first place.
Since Gungnir is capable of destroying even the Etrian Yggdrasil Core, it has a blast radius of dozens of miles and is able to penetrate at least a few miles underground at full power in order to destroy a calamity such as the Core. It is also suspected that Gungnir contains nuclear capabilities since Frederica mentions that the area destroyed by its blast will be unable to recover from the destruction.
The only time the Gungnir is actually seen in action is if the player fails to stop it's countdown in Gladsheim when M.I.K.E. activates the weapon against the protagonist's will, destroying the entire area in a whiteout and causing a game over.