The Stalker is a creature introduced in Borderlands 2. It has a stealth ability that prevents it from being seen by the naked eye, made possible due to a special organ all stalkers have known as the Sachs' organ. This organ, located within the vestigial wings of the creature, stores an electrical charge generated by solar plates on the creature's back that convert sunlight into electrical energy. While hunting or hiding, this organ is used to generate an electrical field, allowing it to blend into its surroundings and disappear (also known as "cloaking"). When the stored energy is depleted or the electrical field is destroyed, their stealth ability is disabled.
Stalkers vanish when they detect prey nearby, then proceed to creep up on their victim. Only when they are close enough to pounce do they reveal themselves, and then they will leap and try to slash at their prey. Their weak shields can be quickly depleted by shock damage, which strips their concealment capabilities until it regenerates, making them very easy targets. Different varieties of stalker attack in different ways - Needle Stalkers prefer to hang back and fling spines from their tails, Spring Stalkers jump in and out of melee range, and Ambush Stalkers tend to pounce into melee and stay there until and unless their shields are depleted. Most stalkers become far more cautious once their shields are down; they will let out a distinctive roar, and then back off to recharge.
It is advisable to find a chokepoint in the terrain for fighting stalkers, as this will reduce their rapid movement, and make them easier to target. Longbow tesla grenades are useful for removing stalkers' shields, and after that, an incendiary weapon with a wide spread pattern, such as the Sand Hawk, Veruc, or Sawbar, will simplify killing them.
- While using Zer0's Decepti0n skill, all stalkers are highlighted even if they are cloaked.
- Cloaked Stalkers emit a purple outline when damaged. When damaged by an elemental weapon, the respective element's colour (incendiary, corrosive, shock, and slag) can be seen in the outline.
- While cloaked, stalkers will not take continuous damage from Tesla grenades, but they will take damage from the initial and ending explosions.
- Cloaked stalkers cannot be Phaselocked.
- Stalker wings are vestigial but still somewhat functional, allowing them to glide.
- Stalkers tend to reproduce asexually. Sexual intercourse between two stalkers is very rare and usually by accident.
- In some areas with stalkers, bandits have hung signs to warn of 'Invisibl Assholes!' which is a misspelling of "Invisible Assholes!" This includes areas without any bandit presence such as The Highlands - Outwash.
- The Sachs' organ that allows stalkers to turn invisible shares its name with the organ used by electric eels for their electric discharges.
- Stalkers recharge their shields with the use of special plates on their backs that convert solar energy into a shield, as seen in the Borderlands 2 official guide book.
- According to Marshall Friedman, stalker skin is used to make boots.
- According to Sir Hammerlock, stalkers have a taste for ECHO recorders.
- During the mission Swallowed Whole, Scooter describes a stalker's digestive system as being "slower than my brother-uncle's brain".
- In Borderlands: The Pre-Sequel, Handsome Jack reveals he plans to develop stealth technology from Stalker research, which would later allow Hyperion Snipers and Hyperion Infiltrators to cloak in Borderlands 2.