The Stalker Form[1] is a Flood Pure Form that specialises in stealth and agility. It is apparently made from the recast bone of dead enemies. Upon heavy damage it will mutate into a Ranged Form or Tank Form.
This form sits low beneath your field of sight 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 adeptness and are capable of tremendous jumps 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.
Killing Stalker Forms
Stalker forms are weak but incredibly fast to leap elsewhere. 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 better job because their spindly physique makes them harder to hit.