Sentinel beam

The Sentinel Beam is a Forerunner Laser-based directed energy weapon. Sentinel Beams are often integrated into Sentinels, from whom the weapon takes its name. The weapon is particularly effective against small Flood outbreaks and energy shields.

Description
The Sentinel Beam was designed to control small outbreaks of the Flood, and was integrated into Sentinels for that very purpose. The weapon projects a yellow-orange or blue beam of energy, which is capable of efficiently cutting through hordes of Flood -- Infection Forms in particular. Continuous contact from a Sentinel Beam can be devastating against any opponent, especially in Halo 3.

Like the Energy Sword and the Fuel Rod Gun, the Sentinel Beam cannot be used by players in Halo: Combat Evolved; it later became usable in Halo 2 and Halo 3. The more powerful blue version of the beam is only available in Halo 2's campaign.

The Sentinel Beam runs on a non-rechargeable battery that runs out very quickly, yet even Sentinels created many thousands of years ago are capable of firing one. It may be possible that a Sentinel Beam attached to a Sentinel is powered by the Sentinel itself. It should be noted, though, that Sentinel Beams taken off of destroyed Sentinels often have some of the battery depleted. Like the Covenant's plasma-based arsenal, it can temporarily overheat if 20% of the battery is used up in one continuous shot; the weapon vents excess heat extremely quickly in an attempt to counter this. In addition to being effective against the Flood, the Sentinel Beam's ability to rapidly drain energy shields also makes it an effective weapon against the Sentinels themselves.

Monitors
Also worth noting is the version of the beam that the Monitor 343 Guilty Spark makes use of. In Halo 3, the construct uses the integrated weapon to destroy a Flood Combat Form, mortally wound Sergeant Avery Johnson, and fight off the Master Chief and the Arbiter. The beam he fires is more powerful than and twice as thick as the usual Sentinel Beam, and is bright red in color rather than orange. It is possible, however, that this beam is not a Sentinel Beam at all.

In the Halo games
In the Halo 2 campaign, the Sentinel Beam is a deadly weapon, easily capable of stripping the shields of an Elite. It possesses a decent rate of fire, amazing accuracy and range, and sustained attacks with the weapon inflict large amounts of damage. The beam's targeting reticule is also reasonably centered, making it easier to aim. The weapon's melee attack is about as strong as that of a Rocket Launcher, and is likewise considerably slow.

Two variants of the Sentinel Beam are seen in the campaign: the standard yellow-orange one in the silver housing, and the more powerful blue beam, which is encased in a gold housing. While the blue version is more powerful, it is also quite rare and will overheat faster than the standard version of the weapon.

The beam makes another appearance in Halo 3, with a few visual upgrades and several technical fixes. Unlike its Halo 2 counterpart, the weapon now has virtually infinite range and fires a much stronger beam that, if held consistently on an enemy, can destroy them within seconds. It is, obviously enough, extremely effective when used against Flood, and will cut down any form -- including the vaunted Pure Forms -- if aimed properly.

In Halo 3, the weapon does not spawn on any unaltered maps, and gametypes cannot be used to force it to spawn; it is only available in Forged map variants. This is likely due to the weapon's increased power and range, which make it devastating in Multiplayer matches.

Beam Variants
The blue Sentinel Beam used by Sentinel Majors is a stronger and more efficient version of the common yellow-orange Sentinel Beam, though it also overheats more quickly and its charge depletes faster.

The beams used by the Sentinels at Onyx were a golden color. Although this variation of the weapon took significantly longer to charge, it is thought to have been capable of destroying a fully shielded Spartan. Furthermore, Onyx Sentinels were capable of cooperating to generate golden beams capable of destroying entire Covenant Cruisers. The power of the beams is one of many differences documented in the Sentinel variant's article; such differences are likely because Onyx Sentinels were guarding something even more important than the Halo Array: the Shield Worlds.

Appearances
In Halo 2, it first appears on The Arbiter, where Heretic Elites used it as a weapon to attempt to stop the Arbiter and his allies from eradicating the Heretic force, in the Gas Mine. Flood Elite Combat Forms then used it on The Oracle, since almost all of the Elite forms were once Heretic Elites. Many of these weapons appear on Sacred Icon and Quarantine Zone, where the stronger blue beam versions were available from the many destroyed Sentinel Majors. It lasted longer than the one in Halo 3.

In Halo 3, It first appears on The Ark, where it can be gained by picking one of the Sentinels being destroyed (by you, although they may attack you if you destroy too many, or the Covenant Loyalists) patrolling the area. Then it was seen on The Covenant, where some Sentinels were tasked to aid the Arbiter and Master Chief in their escape from the Flood in the Ark's Citadel. When Master Chief, the Arbiter and Johnson arrive on the new Installation 04 (II), they had to clear the Flood blocking their way to the Control Room with the assistance of Sentinels. When one of the Sentinels is destroyed, the Sentinel Beam will remain intact. When the monitor 343 Guilty Spark was destroyed, and the Halo ring activated, Sentinels turned on the escaping Spartan and the Arbiter, which caused several casualties and left their Beam weapons collectible to them. The energy of this weapon was drained when using rather than the one in Halo 2.

Campaign

 * Ironically, it is a very good weapon against Sentinels in both version of the Halo trilogy.
 * On the Halo 2's campaign level Quarantine Zone, provide two of your Elite allies a Sentinel Beam, and have them board the riding seats of your Spectre. Since NPCs can't run out of ammo, they make excellent weaponry. Sadly, further in the mission, if you manage to keep both Elites and Spectre intact to that point, you have to leave the Spectre.
 * In Halo 3, the Sentinel Beam performs greatly against the Flood, as Infection Forms will burst if they are even slightly touched by the beam. Human Flood combat forms will die instantly, though Elite and Brute forms may take longer to kill. Aim for the Infection Form embedded in their chest and you'll kill it. Pure Forms are tougher due to their evolved forms; Stalkers and Ranged Forms will die by the time the weapon overheats whereas the Tank Forms will require two long blasts to take down from the stomach.
 * In Halo 3, the Sentinel Beam is very useful for taking down a Brute's Power Armor quickly and effectively. It is also great for taking down Jackals and their shields as well as Grunts' armor, but it may overheat.

Multiplayer
The Sentinel Beam is quite rare in multiplayer games, and if available, it can be a powerful weapon in the hands of the well-trained.

Halo 2

 * This is best used in a small map like Warlock. Also good for Headlong as long as you have other more powerful weapons with you.
 * If the Sentinel Beam is the only weapon you have, conserve ammo and rely on meleeing as much as possible. It has a quick and effective melee. Only fire the weapon when your target is far away, because ammo goes fast. Melee in close quarters, and avoid deadly crossfire. Replace it as often as possible.
 * A Sentinel Beam is useful against unshielded foes, so a grenade to take down the opponent's shields followed by a burst from the Sentinel Beam can quickly kill an opponent.

Halo 3

 * During a game of Infection, the Sentinel Beam is an effective way to rid yourself of chasing Zombie forces, but take care as to where you aim, since the beam is easy to spot and is likely to give your position away if the shot goes astray.
 * When loading up vehicles, a Sentinel Beam in the passenger seat is an incredibly deadly tool. The most flexible location is in the back seat of a Mongoose, although the passenger seat of a Warthog or Prowler is also useful and advantageous to the match. The advantage of vehicle-bound Sentinel Beams is that it can 'clothesline' foes.
 * Steady shots can make the Sentinel Beam a great anti-sniper weapon. Fire at the sniper and then sweep the area. You will eventually meet your target, and they will have a difficult time evading the beam.
 * The Sentinel Beam has an extremely large power consumption rate, about 5% per second, so always carry a backup weapon and try to save the Beam for emergencies.
 * The Sentinel Beam was not available in any Matchmaking playlists, until it was featured on certain map variants used in the Bungie vs World playlist. Its exclusion from regular matchmaking is likely due to the weapon's power.

Trivia

 * The Automated Turret uses a similar beam to that of the Sentinel Major from Halo 2.
 * In Halo 2, there is a lone Sentinel that fires Needler rounds and drops a Needler when killed. This was meant to be a test when Bungie was experimenting with different weapons for the Sentinels. None of these new weapons made it into the final version of the game, but the lone Needler Sentinel was overlooked and not deleted from the final product. It is not to be considered canon.
 * The Sentinels on Onyx have beams with temperatures as high as 15,000 Kelvin.
 * In Halo: Combat Evolved, this weapon was not able to be wielded. When a Sentinel was destroyed, the weapon was either destroyed with the body, or simply too attached to be used.