Constructor

Constructors are small devices that mainly just repair damage to any Forerunner structure they find. The Sentinels will come to their aid if their are attacked, as they have no weapon systems of their own.

Role
Constructors are devices similar to Sentinels in purpose, that mainly repair damage to any Forerunner structure they find, using weak energy beams similar to Sentinel Beams.

The Sentinels will come to their aid if they are attacked, as they are ill-suited for combat. However, their beams will damage someone if they get caught in the path (on legendary their beams can kill in seconds but still not enough for complete and devastating death). They also can use their beams to lower the shields on pistons, allowing them to move throughout the Sentinel Wall quickly. Constructors are encountered only during the level Sacred Icon & the Quarantine Zone, but they can be seen on The Oracle.

When they appear in the Level known as the Quarantine Zone, they appear at the end of the level where the player fights a Flood controlled Scorpion Tank and a Flood controlled Wraith Tank. Along with many Flood using Covenant Ghosts. Two Sentinel Enforcers fight against the Flood near a downed Human Pelican dropship. During all this chaos, if the player watches one of the large and deep trenches, a huge cloud of Constructors fly off into the sky.

Trivia

 * The Sentinel Constructor is similar in appearance to the Covenant Plasma Pistol.
 * It is rumored that in halo3 they themselves may be used as guns