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.

The most effective weapon against these Flood types is the Flamethrower.

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 usually shoot it until it stops moving.

Pure Ranged Form
Ranged forms can usually be found clinging to ceilings. 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.

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 trememdous amount of damage, and can kill you in one hit in higher difficulties.