The Lance of Longinus is a giant red spiral spear with two piercing tips. It is believed to have been found at the South Pole, and is used inside Terminal Dogma to impale the crucified form of Lilith, the Second Angel.
The Lance of Longinus has many mysterious properties, including the ability to change shape as needed (such as converging into a single-tipped spear when thrust at a target). It also has the unique ability to penetrate AT Fields, making it particularly effective against the Angels and the Evangelion Units.
The PlayStation 2 video game Neon Genesis Evangelion 2 describes the Lance of Longinus as a type of life form with a will of its own, and capable of independent movement. It is extremely powerful, able to pierce all AT Fields and place a Seed of Life (the progenitor entity who holds the power of eternal life), into suspended animation. The lance is the reason why a progenitor entity does not reach god-status, and possibly the reason why Shinji Ikari is able to control and reject the Instrumentality process as seen in The End of Evangelion.
The First Ancestor Civilization prepared the Lance of Longinus as a counter-measure or fail-safe in the event that a progenitor entity did not follow its goals. Each Seed of Life would therefore contain such a device. Lilith's counterpart however was lost during the First Impact, and Adam's Lance of Longinus is eventually recovered from Antarctica to control her.
The Lance of Longinus is of critical importance to SEELE and their version of the Human Instrumentality Project, and is therefore not used as a weapon until the desperate battle against Arael, the Fifteenth Angel. As a result the lance is lost in lunar orbit and becomes irretrievable. It eventually returns during the Third Impact as seen in The End of Evangelion, merging with Evangelion Unit-01 to form the Tree of Life.
SEELE's Evangelion Units
Based on Neon Genesis Evangelion Platinum: 06 DVD Booklet
Based partially on Neon Genesis Evangelion 2 for PlayStation 2