Vahki were mechanical order enforcement units which patrolled Metru Nui. There were six models of Vahki: Nuurakh, Zadakh, Rorzakh, Vorzakh, Keerakh, and Bordakh. They were put into operation after the failure of the Kralhi experiment. Vahki received recharges of power in their hives, which were located throughout Metru Nui. The primary purpose of a Vahki was to maintain order. This involved anything from containing a Rahi rampage, to pursuing a lawbreaker, to making sure Matoran worked when they were scheduled to do so. Vahki communicated by speaking the Matoran language at an extremely high speed. Turaga Dume's chambers contained technology which could slow the speech down enough to be understood. Vahki were capable of three modes of transport: flight, walking on two legs, and walking on four legs using their tools as forelegs. They were equipped with stun staffs, nonlethal devices which acted on the mind rather than the body of a target. Their purpose was to correct behavior without damaging valuable workers.
Vahki had a spider-like shape. Vahki enforced the law and kept order in Metru Nui. Vahki traveled in squads led by a squad leader. They had stun staffs that needed to be charged before use. Vahki could fly. No Matoran could hope to outdistance a pursuing Vahki. Vahki were incapable of speech.
In their dormant state, Vahki occupied a circular hive, with one Vahki monitoring each direction. Though they appeared to be completely inactive, Vahki sensor apparatus never fully shut down. Any sound or movement was instantly detected. When Vahki traveled, they moved in much the same way, always with an eye turned toward the flanks and rear. Vahki had sensitive hearing.
Vahki could fly. They could transform from bipeds to four-legged creatures, using their tools as their front legs, and then fly while in this form.
Metru Nui was confronted by many dangers: rampaging Morbuzakh vines, Matoran vanishing from their workplaces, strange beings claiming to be Toa Metru, and so on. The Vahki order enforcement squads existed to keep the city safe for the Matoran citizens. They would watch over the Matoran while they worked, played, and rested to ensure that they were always safe, secure, and where they were supposed to be, whether they liked it or not.
Metru Nui was patrolled by Vahki order enforcement squads, mechanical beings programmed to preserve order at any cost. Vahki were equipped with Kanoka disks and special stun staffs. Operating from central task force hives in each metru, the Vahki ensured that Rahi rampages were contained and that Matoran did not wander away from their jobs. The Vahki were always around to look after the Matoran, whether they liked it or not.
Vahki were assembled in the Fields of Construction in Po-Metru. The original design for the Vahki came from an Onu-Matoran named Nuparu. Vahki parts were forged in Ta-Metru. Then, the Po-Matoran brought these mechanical beings to life by assembling them in Po-Metru. There were two separate assembly areas for Vahki: one to fit the larger parts together, and one to install the clockwork mechanisms into the skull casing. These steps completed the construction process for a Vahki. Vahki were created with very simple behavior patterns installed. Their job was to maintain order, whether that meant stopping Rahi from rampaging through the city or making sure Matoran stayed on the job during their work shift. Vahki were capable of understanding Matoran, but they communicated only through ultrasonic signals. Turaga Dume's chamber in the Coliseum was fitted with technology that translated these signals into speech.
There was an old joke in Metru Nui: "Don't look back, a Vahki might be gaining on you." Vahki lived for the chase. They could not be argued or reasoned with. A Matoran lawbreaker had only two choices: surrender or run. Choosing to surrender would result in the Vahki using their stun staffs to make the Matoran either more willing to work or less able to cause disorder. Fleeing would delay the inevitable for a while, but eventually the Vahki would track down their prey.
Vahki were capable of three different modes of transport. They could walk on two legs; they could walk on four legs, using their staffs as forelegs; or they could shift into a more aerodynamic mode for flight. Most Vahki were not good swimmers. Bordakh were the only ones designed to withstand time underwater, and even they avoided it.
In addition to the six models of Vahki active in Metru Nui, the Po-Matoran constructed two special types of order enforcer: the Kraahu and the Kranua.
The sporting arena in the Coliseum was also used for Vahki training exercises.
Most Vahki, with the exception of Vorzakh, seemed to enjoy chasing lawbreakers.
Vahki captured Rahi found in Metru Nui which were taken to the Archives.
As of mid-story year 2004, 5,000 Vahki inhabited Metru Nui.
Every city, even Metru Nui, had its share of crime. Part of the job of the Vahki was to track down and pacify lawbreakers, a task they carried out with great efficiency. Just a few of the major types of criminal activity the Vahki dealt with regularly were smuggling, theft, disk dueling, and forgery of Kanoka disks.
There was a Vahki central task force hive for each metru. Matoran who planned to travel between metru on their vacation were advised to have their supervisor contact the Vahki central task force hive for their metru. Failure to do so could result in one being stopped by Vahki in other metru for being away from work during one's shift. Nothing ruined a vacation faster than a blast from a stun staff.
Vahki had a central mechanism governing their intelligence.
Vahki were rarely, if ever, surprised by anything. After years of tracking down and subduing Rahi of all sorts, the order enforcement squads were experienced at handling almost any situation. Add to that foiling the ingenious attempts by some Matoran to try and take unscheduled vacations from work – one Ta-Matoran, Takua, practically rated an entire Nuurakh squad to himself – and Vahki could safely be said to have seen it all. Vahki were trained to track, apprehend, and pacify; they were not accustomed to their targets coming to them.
Like the Vahki they carried, Vahki transports never wore out.
Vahki were designed by Nuparu and constructed in Po-Metru.
When the Metru Nui power plant was destroyed by Makuta, a power surge struck the hives and destroyed most of the Vahki. Those that survived were altered for the worse. Their speech centers became slow enough for other beings to understand them. Their stun staffs became incredibly destructive. In addition, their fundamental mission was warped: they now believed that living things created disorder, and that therefore the way to maintain order was to eliminate anything that lived. Makuta used the Vahki to capture Matoran, guard the Prison of the Dark Hunters, and battle the Toa Metru. After the Vahki's transformation, small squads continued to patrol the city, often clashing with Visorak. Eventually the Visorak destroyed them all.