Shifter

The Shifter is the most common variation of Flood pure form.

Background
Flood Shifters are pure forms that possess the ability to alter their configuration in response to the Flood's immediate needs. Shifters are created for the purposes of direct assaults and the support of existing combat forms. Shifters are an undifferentiated mass of Flood Super Cells that can spontaneously adjust into specialized functions in order to accommodate their present purposes, but they generally transition between three separate configurations during combat, each form serving to counter a particular threat in any given situation. Shifters are most often found near newly established Flood hives, synergistically using their native-born environment to fiercely protect the lair from any perceived threat until larger pure forms can be created.

Stalker
When mobility is needed the Shifter will transform into a stalker configuration. In this form, their body sits horizontally and it possesses two powerful hind legs with long toe-like appendages that grant it the ability to rapidly traverse all manner of surfaces, including vertical walls and ceilings, as well as a pair of front limbs ending in powerful claws that it to defend itself if an enemy closes proximity to it. Shifters in this form tend to avoid direct combat, instead seeking to place themselves in a position in which they can transform to one of their other forms to more effectively engage opponents.

Ranged
When facing a distant threat, the Shifter will place itself in an advantageous position before transforming into a ranged configuration. In this arrangement, the Shifter's body resembled a large pod-like creature sitting atop two squat hind legs and a single multi-pronged forelimb. The creature's abdomen segment rests atop its body and is covered in razor-sharp, calcified spikes of which it can barrage enemies from afar. In this form, the Shifter cannot move, but it can quickly transform into another configuration if it needs to relocate.

Tank
For frontal assaults, the Shifter can transform into a hulking tank configuration, balancing the mobility of the stalker with the aggression of the ranged form. This form of the Shifter possesses enormous scythe-like forearms that it can use to inflict devastating damage on its enemies at close range. While this form is generally quite slow, lumbering forward on its ungainly, trunk-like legs, it is capable of short bursts of speed with the aid of its forelimbs in order to close the distance to its target. Tank forms have also been observed to have the ability to spew up torrents of Pod infectors from their 'mouths'. While not considered any more lethal than other pure forms, Shifters in this configuration nonetheless strike fear in the heart of any foolish enough to intrude on their domain.

History
During the Forerunner-Flood war, Shifter forms were among the myriad of Flood forms that were utilized over the course of the galaxy-wide conflict. Following the firing of the Halo Array, a number of Shifter forms were kept in containment on several of the Halo rings, including Installation 04 and Installation 05.

During the Battle of Installation 04 in September 2552, many Shifter forms were able to escape from their containment following the accidental release of the Flood by Covenant forces. Primarily utilizing their tank configurations, large numbers of Shifters were encountered by both Covenant and UNSC troops, including Fireteam Raven, during the later stages of the conflict.

Following the large-scale Flood outbreak at Installation 05, and their subsequent infestation of High Charity in November 2552, the former Covenant Holy City was converted into a Flood Flood hive. With access to enormous amounts of biomass from High Charity's millions of consumed inhabitants, the Flood were able to generate pure forms, including Shifters. A number of Shifters were among the Flood forms carried to Earth on board the infected CCS-class battlecruiser Indulgence of Conviction, and would soon spread across the city of Voi following the ship's crash landing. Many were neutralized by UNSC and Sangheili forces spearheaded by Spartan John-117 and Arbiter Thel 'Vadam during their push to the cruiser's crash site. Any remaining Shifters were destroyed during the glassing of the city by Sangheili warships at the conclusion of the battle.

During the Ark Conflict, following the arrival of High Charity at the installation, a pair of Shifters infiltrated the citadel where the Prophet of Truth was preparing to fire the Halo Array, confronting John-117 and Thel 'Vadam in their tank configurations. Rather than attack the human and Sangheili, the Gravemind spoke through the Shifters, promising to assist the duo in their assault on the final bastion of Covenant defenses as they both sought to stop the firing of the Array. These Shifters, along with Pod infectors and combat forms, aided John-117 and 'Vadam on their push through the remaining Covenant troops, ultimately stopping Truth before turning on the Spartan and Sangheili once the threat of the Array was neutralized. Large numbers of Shifters resided in the remains of High Charity and fought against John-117 during his mission to rescue Cortana, but they ultimately failed and High Charity was destroyed by the Spartan. In a last-ditch attempt to prevent John-117 and Thel 'Vadam from firing the newly-constructed Installation 08, the remains of the Gravemind launched waves of Flood dispersal pods containing Shifters and combat forms onto the surface of the ring, but they once again failed and were obliterated along with the rest of the Flood when the Halo was fired.

List of appearances

 * Halo 3
 * Halo 3: ODST
 * Halo Legends
 * Origins
 * Halo: Combat Evolved Anniversary
 * Terminals
 * Halo: Fireteam Raven