Containment shield

The containment shield was an energy shield wall within the Sentinel Wall around Installation 05's Library, built by the Forerunners. It helped contain and enclose the Flood inside the Quarantine Zone, and keep out anything that could sustain the Flood. It also appears to be a sort of energy field that doesn't allow anything through either way. It had to be lowered and deactivated in order for Covenant forces to enter the Library and recover the Activation Index.

History
During the Battle of Installation 05, the containment shield kept UNSC forces stationed on Delta Halo (as well as the Arbiter) from reaching the Index. In order the lower the Containment Field, the Arbiter fought his way through the Sentinel defenders and deactivated the generators for the shield.

However, the generator couplings were protected by a Sentinel Enforcer. After destroying the Enforcer, the Arbiter finally deactivated the shield. This allowed access to the Quarantine Zone by the Covenant and the UNSC, giving the Flood hundreds of new hosts. The Flood was released onto the rest of Installation 05. As a result of this, the was taken over by the Flood and flown into the Covenant city of High Charity.

Trivia

 * In Halo 2, the Containment Shield was observed during the level Sacred Icon.