Flood pure form

Pure Form is a new type of Flood that is featured in Halo 3. As implied by their name, Pure Forms are not the product of infecting a biological host, but are instead composed entirely of the Flood super-cell biomass like the small Infection Forms, using a calcium-based exoskeleton for protection and support. As such, they are also different from their parasite-ridden counterparts in that they are not made out of weak, rotting flesh, but a hardened carapace of recycled/recompiled bones, muscles, and biomass that you cannot dismember or "sterilize" like you can with the parasitic Combat Forms, even with weapons as powerful as the Flamethrower. However, once you do kill them, they cannot reanimate, much like non-Flood enemies. There are three types of Pure Forms, Stalker Form, Ranged Form, and Tank Form, and any one Pure Form can mutate between these sub-forms at will, with the Stalker seeming to be the "base organism" that the other two forms mutate from. (i.e., if a Tank wanted to become a Ranged Form, it would first have to turn into the Stalker)

Weaponry effective against these Flood types includes the Flamethrower, the Energy Sword, and the Needler. The Plasma Rifle can be effective at very long ranges, even on Legendary. For Lower Difficulties melees with Brute Weapons have considerable damage often only needing one or two hits. Incendiary Grenades are on of the most effective weapons against the Pure Form. Taking down any Flood form with one stick and incurring subsequent damage to any nearby Flood.

Pure Stalker Form
Stalker forms are very agile and can jump large distances. They tend to crawl on ceilings and walls before leaping at you. Generally speaking, the Stalker Form, even on the Normal and Heroic settings of the game, are too fast to be aimed at for any significant period of time. However, should one drop down from the ceiling, the best policy is to melee it, as a melee is a one hit kill. Don't waste ammo trying to shoot it, they are very resistant to bullets. They attack far more rarely than the other two forms. If you leave one alone for some time, it will either become a Tank or Ranged form.

Pure Ranged Form
Ranged Forms can usually be found clinging to ceilings or walls, though unlike the Stalker Form, they rarely move from a given spot. They can fire sharp projectiles at you, without the need of a weapon. These are slightly more tricky to kill due mainly to the fact that they can have you pinned down in a position for a very long time. If they are left alone for some time, they will change back into a Stalker form, get a better angle, and become a Ranged from again. The general consensus is, however, that if you have access to a Needler at a medium distance and fire a volley, they go down easily enough. Plasma Rifles tend to work just as well at closer distances. Also note that this Pure Form may shield its self by curling up, it is then almost invulnerable (the Flamethrower, Energy Sword, Gravity Hammer, general explosives, and sustained Spiker fire will still kill it). A Brute Shot may be the best way to knock a Pure Ranged Form off of a wall by means of one or two shots. Please note that when changing form it is easiest to kill, seeing how it can't move while changing and it seems to have the health of the Stalker Form.

Pure Tank Form
Tank Forms are large and easily spotted. Unlike the other Flood Pure Forms, they can not climb on walls and ceilings. These Flood can take a tremendous amount of damage and can kill you in one hit in higher difficulties. They also occasionally spew Infection Forms from their 'mouths'. Usually one or two hits from an Energy Sword will kill them. A Flamethrower can kill a Tank Form with only a small amount of flame used. A Plasma Rifle has also proven effective to a certain degree. Also, the melee of a Brute Shot can be very effective, especially if used in a "hit and run" style, with the player rushing up, meleeing, and then retreating (2 hits are needed on Legendary, or a shot and a melee). This tactic is very effective when Tank Forms are supporting a large number of Combat Forms, as it frees up much needed ammo for fighting other Flood troops. Also it is helpful as the Brute Shot melee has infinite "ammo," so one can essentially take down a whole horde with clever melee tactics. However, please note that when changing form it is easiest to kill, seeing how it can't move while changing and it seems to have the health of the Stalker Form. When use of a Shotgun is necessitated, get close and aim for the target's "mouth".

Trivia

 * The Energy Sword is the only weapon that can kill a Tank Form in one hit.
 * It is theorized that the Flood Juggernaut, a Flood Form cut from Halo 2, was an early render of the Halo 3 Pure Form. The Drinol, another deleted species from Halo 2, may have served as the inspiration for the Tank Form.
 * If a Pure Form is stuck by either a Plasma Grenade or Spike Grenade while mutating, the grenade will be "absorbed" and no explosion will occur.