The Lance of Loginus or Spear of Loginus depending on your translation, was a very important part of initiating the 3rd Impact.

It also was capable of breaking though Absolute Terror Fields which led some people to believe that that Lance possess a soul. This is because generally only souls produce AT Fields and AT Fields can only be penetrated by Anti-AT Fields or other A. T. Fields.

There is an alternate theory which suggests that the Lance may in fact contain an S² Engine which would account for both its ability to pierce AT Fields and for its requirement for use with Lilith to initiate Instrumentality.

Update 19/04/01

"A giant spiral-shaped lance which is capable of piercing even an A. T. Field. The tip of the Lance is split into two branches, but when thrown these two branches converge into one and penetrate the object. The Lance was first used in the battle against the 15th Angel. It destroyed the Angel in a satellite orbit, but the Lance then continued on to reach the Moon and became irretrievable. However, it appears that the original use of the Lance is not as a weapon, but that it is a necessary item for Instrumentality (Complementation) using Lilith. Incidentally, Longinus was the name of the soldier who used his lance to pierce Jesus as He was crucified on the cross. This Lance is stored as a relic in the Cathedral of St. Augustine in Rome."

Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.