From Halopedia, the Halo wiki
The Promethean Knight serves as a mechanized Forerunner Promethean warrior created by the Ur-Didact for use against the Flood and the player in all installments of the Halo series that feature them. They direct Promethean Crawlers, regarding them as cannon fodder.
Knights are always equipped with Forerunner weapons, with the Suppressor and LightRifle being their default armament. They are also armed with lightblades, which they can use to engage enemies at close range.
At range, Knights engage the player with their weapons and slash the player with their lightblades at close range. In addition, Knights can jump towards the player to get up close and attack with their lightblades. In Halo 5, Knight Commanders and Knight boss variants utilize the charged shot of their Incineration Cannon when they jump, rather than attack the player with their blade. They also sometimes deploy Watchers from their carapaces; the Watchers provide protection with Hardlight Shields. As with Elites, Knights can easily be defeated with the Noob Combo using the Plasma Pistol, although this only works on either levels with both Covenant and Prometheans or if the player uses a loadout with a Plasma Pistol in Spartan Ops.
Knights can also teleport away from and towards the player to confuse or surprise them. They are most vulnerable right after teleporting in and are otherwise invulnerable while teleporting. In addition, Knights can enter "rage mode" and aggressively charge towards the player.
If a Knight loses its shields, it will likely roar and be enraged, during which it exposes its face. Shooting it there with a precision weapon will kill the Knight instantly. However, unlike Elites, Knights are more likely to fight back at close range with their deadly lightblades, which can prove fatal on Legendary difficulty.
Once a Knight dies, it leaves behind a small ball of swirling light for a brief period of time. A Watcher can come and revive the Knight from this ball of light, so the player should kill the Watcher too.
Knight Battlewagons can detect the player even through walls and are usually armed with the Scattershot.
Knight Lancers are more dangerous as they are armed with the deadly Binary Rifle, which can instantly kill the player on Legendary difficulty, and they can perform a zig-zag charge against the player, although this can be avoided without too much difficulty,
Knight Commanders are the most dangerous Knights, as they are usually armed with the Incineration Cannon, which can instantly kill the player on any difficulty except Easy. In a similar manner to Hunters in Halo: Reach, Knight Commanders tend to jump and fire their Incineration Cannon at the ground, relying on the weapon's massive splash damage. However, they are also the rarest type of Knight, as they only appear in the missions Shutdown and Midnight.
Halo 5: Guardians
Changes from Halo 4 to Halo 5: Guardians
Despite being more armored, Knights can be considered less dangerous than in Halo 4, as they now lack energy shields and are no longer capable of teleporting nor throwing grenades. In addition, Knights are considerably rarer as Promethean Soldiers mostly replace them on the battlefield.
To quickly kill one, shoot one at either one of its exposed side cores and then headshot it with a precision weapon once it exposes its head and "screams". If the Knight is not killed within this time, it will quickly draw its head back in again, forcing the player to destroy its last core to expose the head a second time. If the player fails to kill the Knight even after destroying the two cores, the Knight's exposed head will remain unprotected, leaving it vulnerable for the remainder of the match until destroyed or healed by a Watcher. However, Legendary and Mythic Knights cannot be killed through this method as they will not expose their head when they scream, forcing the player to destroy both cores to expose a weak point on their backs, which will cause the Knight to suffer more damage when shot from behind. They can still be assassinated, though.
In the campaign, players must exercise caution when fighting Knights armed with Incineration Cannons, as they have no qualms about firing charged shots from their weapons, which can instantly kill a player character and make them unrevivable until their next respawn.
Warzone and Warzone Firefight introduce Legendary and Mythic versions of Knights, all with significantly more health. The Legendary ones appear as orange with teal highlights, while the Mythic ones are crimson and bright silver with brighter red highlights. Legendary Knights also appear as bosses, sometimes armed with the River of Light Incineration Cannon that fires seven mortar shots for its charged shot. Mythic Knights, including Endurance-of-Will, are even more powerful and dangerous, as they can be armed with the deadly Heartseeker, an advanced Incineration Cannon that fires tracking charges, which can especially spell devastating results on players either on the ground on in slow-moving vehicles like Scorpions, even more so with Mythic Knights being able to fire them more quickly than players can. Despite this, Heartseekers when used by Knights have weaker tracking behavior than when used by Spartans, making Mythic Knights' Heartseeker projectiles slightly easier to avoid by comparison if the player is in a vehicle. Legendary Knight Battlewagons use the Loathsome Thing, while Mythic Knight Battlewagons use the Didact's Signet.