Stalker Form

The Stalker Form is a Flood Pure Form that specializes in stealth and agility. It is apparently made from the recast bone of dead enemies. It will frequently mutate into a Ranged Form or a Tank Form when the need arises

This form sits low beneath your field of view and their attacks can go widely unnoticed until you look down. They are able to crawl on walls and ceilings like a spider with relative ease and are capable of tremendous jumps much like the normal parasitic combat forms. A possible flaw in the Halo 3 AI is that it has been observed to rarely attack the player, and normally prefers to mutate itself into one of the two other Pure Forms. It only attacks the player on the level "Cortana", otherwise, it will not attack the player in other levels.

How to Defeat Stalker Forms
Stalker forms are weak, but they are incredibly fast and agile. If you can get up close, a single melee attack from any weapon can usually bring one down. Any projectile weapon will do, but grenades do a much better job, because their spindly physique makes them harder to hit. If you hit them with an Incendiary Grenade or a Flamethrower, they will instantly jump away and scream. Then they will die after a few seconds of burning. Although unconfirmed, once a Stalker is set on fire, they will die once they hit the ground. For the "second" time. If a Stalker is in midair, and is only hit once by the fire, they will merely die once they hit the ground. The name "stalker" is represented quite well on both Cortana and Floodgate as they blend in quite well with the surrounding flood ridden walls.