Physically, the Spear of Longinus resembles a long, red bident on an Evangelion scale. Its structure is inherently double-helical, with the majority of its length being straight and tightly interwoven. At one end, it unwinds and forms two massive tines.
The Spear is considered to be a weapon of "god-like" power. N² weapons have enough destructive potential to bring minor amounts of damage upon some of the weaker Angels; and Evas can use armaments against Angels after nullifying the Angels' A.T. Fields with their own. However, the Spear of Longinus is capable of piercing the active, unneutralized A.T. Field of any Angel or Evangelion, and entirely by itself.
While the bident is the Spear's default shape, it also has metamorphic capabilities. When thrown at the Angel Arael(22), the tines wrap together to form a seamless piercing object. Upon encountering Arael's A.T. Field, the "flaps" of the Spear open up, revealing white, gill-like structures that seem to aid the Spear with self-propulsion under conditions of extreme resistance. Finally, the Spear's bident shape is perpetually altered by Eva-01 in Episode 26’ when she separates the two helices, and the Spear takes on a form reminiscent of a horizontally elongated infinity symbol, ∞.
The Spear is not of Terran origin, having been created by the First Ancestral Race, an ancient extraterrestrial humanoid species. It is also not the only Spear of Longinus created: Every Seed of Life, including both Adam and Lilith, was sent with a counterpart Spear. At least one established function of the Spears is that of a "security device" capable of safeguarding the First Ancestral Race's plans should they somehow be compromised.
Spears of Longinus are technically alive and are even said to have wills of their own. The Spear featured in Evangelion belonged to Adam, whereas Lilith's is believed to have been lost or destroyed during First Impact.
Pre-2015
First Impact
Adam, contained within the White Moon, was the first Seed to land on Earth, at least 4 billion years ago. Before Adam-based life could be established, Lilith crash-landed on Earth within the Black Moon, her Spear apparently being destroyed in the process, and caused the event now known as the First Impact. Due to an unspecified law in the First Ancestral Race's plan, two Seeds are not meant to co-exist on the same planet, and, resultantly, Adam's Spear -- as the only one remaining on Earth -- took the initiative to subdue its own counterpart Seed. Adam remained in suspended animation for billions of years while Lilith-based life was allowed to develop and prosper.
The Spear of Longinus impaled in Adam's back, as found by the Katsuragi Expedition
Second Impact
In the year 2000, the Katsuragi Expedition unearthed Adam and the Spear in the Antarctic. The Spear was removed, briefly relocated to the Dead Sea region, and then shipped back to the South Pole in preparation for the Contact Experiment. After the experiment led to unforeseen consequences, the team attempted to wield the Spear against Adam to "reseal" her, but they failed. In classified video footage of the incident (21’), scientists can be heard shouting off-screen that the Spear needs to be 'pulled back', and that it's "sinking in". Second Impact could not be averted, and, ultimately, the Spear appears to be responsible for causing Adam's retroversion to an embryonic state (Chronicle).
Present
The Spear first appears in the series in Episode 12 under wraps as it is transported from the Antarctic to Nerv on the flight deck of an aircraft carrier. It reappears at the end of Episode 14 as Rei is ordered by Gendo Ikari to go into Terminal Dogma to insert it into Lilith, the result of which is seen in Episode 15. It remains there until Episode 22 during the battle with Arael, upon which Rei once again descends to collect it from Lilith in order to destroy Arael, which was floating in outer space high above Earth's atmosphere. Rei used Evangelion Unit-00 to hurl the Spear of Longinus like a javelin into Earth orbit, and kill the Angel. The Spear exceeds escape velocity, and is lost from Earth's orbit, becoming trapped in Lunar orbit, and eventually landing on the Moon.
Seele is able to create nine copies of the Spear of Longinus for use by their Mass Production Model Evangelions. The copies have most of the original's properties, such as its size and dimensions, along with its ability to pierce A.T. Fields, although they are bluish grey in color rather than red. Also unlike the original, they cannot be used to initate Instrumentality with Lilith herself. Even after the original Spear was lost, Seele felt they still had a chance, albeit more difficult, of initiating Third Impact, by using the replica Spears and Eva Unit 01 (itself created from Lilith), though it is possible that even Seele wasn't entirely sure if this would be sufficient (this remains unknown as the original Spear then returned to Earth).
The metamorphic capabilities observed in the replicas exceed that of the original: When the Eva Series are first sortied in 25’, their Spears are initially disguised as massive, double-edged blades that bear no resemblance to their native forms. The weapons' true nature is revealed when one comes in contact with Eva-02's A.T. Field and, within seconds, responds by morphing instantaneously into its Spear shape.
Officially, the "double-edged blade" variation of the Spear copies goes by two different names. The script refers to them as 重い槍 (omoi yari), or "Heavy Spear". However, this term is rarely observed elsewhere, and 大剣 (ooken), or "Giant Sword", is seen in its stead.
Notes
The Spear is named after the Lancea Longini (aka "Lance of Longinus", "Spear of Destiny", "Holy Lance") a weapon that, according to Christian tradition, was used by the Roman soldier Longinus to stab the crucified Jesus Christ in the side to confirm his death at his crucifixion. Bearing the blood of the Son of God was said to, consequently, bestow the spear with supernatural properties, making it a much-sought-after artifact (in a manner similar to the Holy Grail). Over the centuries there have been many reports of the Holy Lance being rediscovered, buried in the Earth, most notably when the Lance was supposedly discovered during the Siege of Antioch in the First Crusade, becoming a rallying point for the besieged Crusaders and leading them to miraculously break the siege against all odds.
"Spear of Longinus"In Japanese, the Spear is called "Longinus no Yari", with "yari" being translatable as either "lance" or "spear". The translation "Lance of Longinus" is utilized by ADV and Manga Entertainment's localizations and is, resultantly, in prominent use by the English-speaking fan community. However, a computer display in 25’ (see figure) indicates that the official English rendition is, in fact, "Spear of Longinus".
After being thrown at Arael by Eva-00, the Spear of Longinus issues a sound identical to an Evangelion's roar. (Episode 22')
When the Spear copies are destroyed, they initially bubble up into LCL-colored masses with little lower bodies projecting out of them (as per Lilith on the cross).
