Surrounding the model in a dark miasma of energy, a shadow field absorbs the energy of any hits inflicted on the model wearing it, making them almost immune to any damage. However, the energy field can become overloaded and cut out. A shadow field provides a 2+ invulnerable save, but if the save is ever failed then the field is destroyed. Shadow fields are rare, so no more than one shadow field may be taken per army. Despite having a 2+ save, the model is still considered as being fleet of foot.
Shadow Field
Source: Codex: Dark Eldar (2002), p. 15