The Flood Juggernaut is a large Flood command form. Fireteam Raven fought against multiple Juggernauts during the Battle of Installation 04. The Abomination key mind form is an evolution of the Juggernaut.
Juggernauts are highly specialized combat forms directed by a seething mass of infection forms linked to multiple hosts. The creature harnesses the hosts' minds to analyze enemy activity and adjust the local Flood strategy accordingly. They have some of the traits associated with full-fledged key minds, including the abilities to synchronize nearby parasites and relay information between distant Flood hordes.
The most significant aspect of the Juggernaut is their enormous size and raw physical strength, capable of impaling armored vehicles with their sharpened tendril-arms and crush unsuspecting victims with their trunk-like legs. These tendrils are capable of picking up a Hunter or impaling a fully-armoured Sangheili, and can be used to batter their victims against nearby surfaces or simply throw them into the air.
Multiple Flood Juggernauts were unleashed by the Flood during the Battle of Installation 04. Following their crash-landing late in the conflict, Fireteam Raven encountered two pairs of Flood Juggernauts battling the forces of the Covenant, and were able to destroy them while waiting for evacuation. During the final Mission to destroy Installation 04, at least five Juggernauts joined the Flood's attack on the downed UNSC Pillar of Autumn, where they battled the remaining Covenant and UNSC forces on the plains below the crash site. Three of the Juggernauts were killed by the focus cannon of a Type-47B Scarab, while the remaining two were killed by Fireteam Raven.
The Juggernaut was originally planned to debut as a gameplay enemy in Halo 2, but was ultimately cut from the game. The form's canon status remained in limbo until it was mentioned in Halo Wars 2's Phoenix Logs and eventually made an appearance as an enemy in the arcade shooter Halo: Fireteam Raven.
Juggernauts were originally slated to appear in the levels "The Oracle", "Sacred Icon" and "Quarantine Zone". There would never be more than one Juggernaut in any given place. Once the player eliminated it, the combat forms that were supplementing it would go "offline". The Juggernaut has four to six infection forms inside its body to control it, similar to combat forms. According to Robert McLees, the Juggernaut was supposed to be "... this uber-intelligent mini boss, but the suggested intelligence was way too high for what design needed to use it for, which was as a big tank".
The Juggernaut was also the conceptual progenitor of the pure forms seen in Halo 3. However, whereas the Juggernaut was to be controlled by infection forms, pure forms are created entirely from Flood biomass.
Halo 2 gameplay
The Juggernaut's model and AI are still present in the game files, allowing the NPC to be added to gameplay via modding, similar to the Huragok in Halo: Combat Evolved. The Juggernaut stands nearly twice as tall as a Spartan or an Elite. It has two large tentacles that can kill the player character with only one hit on Legendary difficulty. The player can survive this hit on lower difficulties, though will often be sent flying through the air. Finding cover is ill-advised, as the Juggernaut will hit it out of the way, sending the object a great distance and killing the player if it connects. They also can jump much higher and farther than regular combat forms can. They often jump high in the air at the player character when noticed, sometimes landing on top of them and killing them instantly. The Juggernaut will also sometimes forget the player character is there remain still for a few moments. The Juggernaut is highly resistant to damage. Powerful weapons such as the shotgun and rocket launcher are all but necessary against it. Standing under the Juggernaut and meleeing it will cause the enemy to recoil - doing this repeatedly can stun-lock the Juggernaut.
Through modding, it has been found that the Juggernauts have a defensive position; stopping and moving their whip-like appendages in front of their body in an X formation. When running low on health, some occurrences have happened where the Juggernaut goes berserk, waving its tentacles around wildly before striking the player with speed. As Bungie never created a death animation for the Juggernaut, when it has ran out of health, it simply freezes in place. The Juggernaut also has a taunt animation, in which it slams its tentacles on the floor and "roars" at the player. 
Although the model, textures, etc. for the Juggernaut are stored in the files of the level "High Charity", there are no spawn points for it. Modders and Halo 2 Vista players can fight against the Juggernaut on the level by switching them to spawn in place of another character's existing spawn point. Code within the files for the levels The Oracle, Sacred Icon and Quarantine Zone indicates these missions were to have had Juggernaut spawns within them, though the level files themselves do not contain the Juggernaut assets. The Oracle would have one Juggernaut hidden in the fog (not able to be fought), and one attacking the Flood research laboratory later on in the mission. Another would spawn in the defense section near the end of Sacred Icon, while six were to spawn on Quarantine Zone - likely due to the abundance of heavy vehicles in the mission. These Juggernauts would have spawned as follows; two in the canyon in front of the destroyed Sentinel manufacturing facility, one inside, one outside the exit and two more at the entrance to the gondola. This would have lead to nine possible Juggernaut encounters in the game, with eight as actual combatants.
List of appearances