Flood pure form

Pure Form is a new type of Flood that is featured in Halo 3. There are three sub forms, the Stalker Form, Ranged Form, and the Tank Form. Each Pure Form can mutate into another, with the Stalker seeming to be the "base organism" that the other two forms mutate from. As implied by its name, Pure Forms are not products of a biological host, but are instead conceived entirely of the Flood like the small Infection Forms. As such, that 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 of recycled/recompiled bones 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 do not come back, much like non-Flood enemies.

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.

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.

Pure Ranged Form
Ranged Forms can usually be found clinging to ceilings or walls. 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. 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 and general explosives will still kill it). A Brute Shot may be the best way to knock a pure ranger 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 spew from their 'mouths' more Infection Forms. Usually one or two hits from an Energy Sword will kill them. 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.