The Z-1500 Automated Systems Drone, also known as an Aggressor Sentinel and by the Covenant as "Holy Warriors of the Sacred Rings," is a broad category of Forerunner-constructed robots designed to suppress Flood outbreaks, construct and maintain Forerunner structures, mine and retrieve raw materials, and protect Forerunner technologies from alien lifeforms. To this end, they possess limited artificial intelligence and boast a wide range of weaponry and equipment in the field.
Following the construction of the Halo Rings, Sentinels were built to serve as the first line of defense against Flood outbreak and external intrusion. The non-organic composition of the Sentinels allowed the Forerunners to battle the parasite without contributing to its numbers. Controlled by their own autonomous AI and their Installation's Monitor, Sentinels patrol the Halo rings and destroy anything deemed to be a threat.
Outside the Halos and other Forerunner installations, the Sentinels were seen only fighting under the San'Shyuum control, as main infantry units, during the Sangheili-San'Shyuum war. However, after the end of the war, barely any operative sentinel survived, and they were replaced, in the Covenant Empire's army, by the Sangheili warriors.
Sentinels are grayish-white in color and have two arm-like appendages with two grapple fixtures at the end of each arm, along with a central "head" and a curved undercarriage containing a Sentinel Beam. The form of a Sentinel may differ depending on the situation of their Installation. Their powerful offensive beam weapons and defensive energy shield make them very effective in combat against intruders or foreign objects that present themselves as threats. Their smaller lasers are used for both repairing damaged Forerunner constructs, and self-repair should the Sentinel be damaged in combat. Sentinels are propelled by both an antigravity unit located on the undercarriage and small engines on the back of the unit. They are capable of traveling at high speeds but cannot sustain long-range travel.
Sentinels are created at Sentinel Production Facilities, massive floating platforms that are located on every Halo Installation. These Facilities produce all types of Sentinels when facing either a Flood outbreak or an external threat to a Halo Installation or Construct. These massive assembly lines at maximum performance can produce Sentinels at a very fast rate to either supplement existing Forerunner defenses or to replace those destroyed for whatever reason. Sentinel Production Facilities are not limited to just the Halo Installations, Production Facilities are located on every Forerunner Construct that relies on Sentinels for guarding, although these facilities are different in appearance and may be either ground or mobile based platforms.
The Forerunners refer to the Sentinels as "our very basic countermeasures," and they are the first automated defense systems employed to assist with a situation and generate a proper response protocol. While Sentinels are strong enough to handle a situation, they are more of a supplement rather than a reliable combat system. The Sentinels' weapons system are limited and not as powerful as some of the other Forerunner weapons employed against the Flood, and as such can only hold them at bay for a limited amount of time.
Because of their limited AI, they are only able to handle situations on a basic scale, which makes them vulnerable to strategic Flood attacks. Because of their limited intelligence, they are not often able to perform evasive maneuvers when facing imminent attack and are usually destroyed. In large overwhelming numbers, Sentinels are more than capable of handling a situation until more permanent measures can be applied.
The Sentinel's energy shield is its primary defense mechanism - once taken down, the Sentinel is susceptible to any type of small arms fire, especially plasma weaponry. When the Sentinel has been damaged beyond repair, the Sentinel explodes violently and releases a powerful EMP blast. This is a strategic last resort, as the blast will disable any nearby electronics, including any weapons the enemy may be using.
- Main article: Sentinel Major
Sentinel Majors are larger, more powerful Sentinel models used for heavy combat situations. They provide backup for Enforcers in the event of a massive Flood outbreak or attacks on the Halo Installations.
- Main article: Enforcer
Essentially heavy Sentinels, Enforcers are equipped with a variety of offensive and defensive weaponry, including Pulse Beams, Missile Projectiles, Sentinel Beams, and dual Energy Shields covering both their top and bottom halves. They are designed to handle major Flood outbreaks but are also capable of defending their Halo Installation from external threats.
- Main article: Onyx Sentinel
Onyx Sentinels both guard and make up the Shield World of Onyx. With the ability to combine themselves in order to increase their overall beam power, as well as a much more powerful individual beam, Onyx Sentinels are formidable opponents.
- Main article: Super Sentinel
Considerably larger than normal Sentinels, Super Sentinels fill the same role of combating outside threats. Like Enforcers and Sentinel Majors, Super Sentinels are only seen on the field in an event of a massive Flood outbreak. They have the unique ability to stun buildings, vehicles, and infantry.
- Main article: Protector Sentinel
Roughly similar in appearance to regular Sentinels, Protector Sentinels occupy varying roles, from healing or shielding other units to offensive capabilities similar to other sentinels.
- Main article: Retriever
Retrievers are massive machines, used to mine and retrieve raw materials and construct large-scale installations, including the Halo Array.
Unidentified Sentinel Variant
- Main article: Unidentified Sentinel variant
The largest Sentinel variant seen to date, this Sentinel variant is able to level entire cities with its weaponry.
- In Halo: Ghosts of Onyx, an Onyx Sentinel approaches a SPARTAN-III and attempts to communicate with him, first in Latin, then in English.
- In the Halo 2 level Quarantine Zone, a glitch causes a Sentinel to use a Needler.
- Chris Hughes created a crablike machine with many animating parts that gave the Sentinel life and character.
- Halo: Combat Evolved (First appearance)
- Halo: The Flood
- Halo 2
- Halo: Ghosts of Onyx
- Halo 3
- Halo Wars
- Halo Wars 2
- Halo Legends
- Halo: Blood Line
- Halo: Broken Circle
- Halo: Cryptum
- Halo: Primordium
- Halo 4
- Halo: Silentium
- Halo: Spartan Assault
- Halo: Last Light
- Halo Infinite
- Halo Legends, Origins
- Halo 4: The Essential Visual Guide, page 133
- Halo: The Essential Visual Guide, page 167
- Halo 2, campaign level The Arbiter - "A Whisper in the Storm"
- Halo Encyclopedia, Chapter 8, page 226/227
- 'Halo: Broken Circle - p.189
- Halo Encyclopedia Chapter 8, page 226
- The Art of Halo, page 44