Stalker

""There is a dark presence hunting down Tenno who have brought death to our enemies. It has a misguided sense of justice and sympathizes with those who seek to destroy our society. Its powers are those of our own and the armor it wears is cloaked in shadow - it must be destroyed.""

- Warframe Lore Thread

The Stalker is an ominous, vengeful figure that infrequently appears during missions, encountered with a level that ranges anywhere between 30 to 70, that will target and attack a single player within the current cell. This enemy will taunt the player with messages in a similar fashion as bosses. For the duration of these messages, the targeted player's HUD will flicker, along with any surrounding lights. After a total of three taunts, the stalker will teleport into the session and proceed to attack the targeted player.

The Stalker's current arsenal is comprised of a bow known as the Dread, a scythe called Hate, throwing knives known as Despair, and an unknown exo-armor that is capable of using power modules from a variety of different Warframes. If the targeted player dies, the Stalker will disappear in a cloud of black smoke. If the Stalker is forced to flee (by damaging his health up to 90%), a bounty of blueprints and more can be obtained.

It is speculated that the Stalker is a rogue Tenno.

Strategy
Stalker attacks with high damage output which can kill an inexperienced player with a few melee attacks. The obvious strategy is to keep moving to avoid his arrows and abilities. The Stalker is similar to Ash because of his speed. He does not reappear in the same mission you are currently playing twice (he can return to the same level later on however). The Stalker targets only one player at a time, but he is also easy to take out if your weapon puts out enough damage to take out his shields. He does not get distracted by Loki's Decoy or Saryn's Molt. In general, he appears to be immune to any Crowd Control - both from Weapons (Stun, Staggering) and Utility Abilities (e.g. Trinity's Link, which won't work on him), although Damaging Abilities will damage him. Therefore, Excalibur's Slash Dash, for instance, can be used to damage him and also escape from an otherwise fatal Melee Combo.

However, it is possible to use Loki's Invisibility, Ash's Smoke Screen or Shade's Ghost cloak to escape certain death for a moment and then run away (unless stopped by a Lockdown or a door requiring two players to open). If you do this, he will get attacked by the mission's enemies just like a player and eventually die. This is especially useful in Infested missions, as a bulk of Infested creatures will be able to mow him down. Another great place to try this is any Corpus mission that has a huge amount of enemies in an outdoor place. Be careful when in a group though, as he can also attack your teammates if he is unable to reach you, and he will usually survive any amount of monsters charging him if you're playing a low level mission.

Another working strategy, although more difficult, is to lure him into a room with breakable windows and shoot them from outside, locking the room down. You can then wait until he dies/disappears (of which you will be notified in the same manner as you were notified of his imminent arrival).

Shotguns appear to be extremely effective, especially with Armor Piercing and/or Shock damage. Given Stalker's favor of melee combat and agility, it can be hard to land anything besides shotgun blasts.

If having difficulty, bring an allied Loki or Mag who can switch places or charge your shields respectively. This can be of huge benefit as an unexpected Stalker can quickly deplete shields and snap actions by Loki or Mag can be life saving.

Another way to survive against stalker is using Rhino's Iron Skin ability and use a melee weapon with a strong charge attack, such as Furax. Be warned though, this strategy requires you to have at least 200 energy to be fully effective due to the fact that in higher levels he has more health and is much harder to take down, and if you run out of energy, you will be defenseless to his staggering melee attacks.

If you are in a match on solo mode, a great way to defeat the Stalker is by jumping on boxes right before he uses Slash Dash. The Stalker will most of the time get glitched into the side of the boxes, making it very easy to defeat him.

If farming for the stalker by boss killing, then going into a low level mission solo, it is recommended to take ash and a high dps weapon. Smoke as soon as he can start to target you, then unload into him. (Can also be done using loki's invisibility)

When in a group, if the stalker is targeting you, just run around near your allies, allowing stalker to keep you as his target, but for your allies to deal all the damage. This can result in some very easy victories against the stalker.

Surviving Stalker Ultimate Ability - Slash Dash (with any warframe)
When Stalker spawns, he first tries to use Slash Dash move on you which can instantly kill even a high-ranked warframe.

Avoiding his Slash Dash move is a must in order to survive the encounter with him.

A nice way to survive this Slash Dash is to get on top of a box, crate, container or anything you need to "climb" on. Be wary that he still can shoot you with his Despair which is less damaging than his Slash Dash, however still fatal enough.

NOTE: update 9.6:

Stalker was tweaked so he won't appear when the person he stalks is on a crate.

In order to countermend that you can stand near a crate and jump on it when he first appears.

Abilities
It uses already existing abilities from other warframes:
 * Ash's Shuriken and Teleport
 * Excalibur's Slash Dash

Quotes
Stalker's list of taunts (one randomly chosen 3 times in the following order):

Taunt 1 Taunt 2 Taunt 3  Player Defeated   Stalker Defeated  Misc
 * (Player's name), you can't run from your past.
 * (Player's name) there is no place to hide.
 * I know your every move (player's name).
 * The blood of (boss' name) is on your hands. Did you really think there would be no repercussions?
 * The murder of (boss' name) will not go unpunished.
 * There is no salvation for your crime against (boss' name).
 * You shall not leave this place!
 * I am your reckoning!
 * Your sentence is death!
 * (Player's name) shall not trouble us again.
 * It is done. (Player's name) is no more.
 * Justice is served. (Player's name) has been executed.
 * What have you...done?
 * No?! This is not... possible.
 * I have failed. (Player's name) lives.
 * I... have failed... this one will remain... unpunished...
 * Your TENNO powers are useless!

Bugs

 * On rare occasions, the stalker will spawn normally, but glitch when he uses his cloaking ability, becoming permanently invincible to damage dealt by your team, being only damaged by enemies. This seems to be associated with the bug below as well.
 * The Stalker can spawn in a defense mission but replacing the pod health bar with his own health. Even if the pod were to be destroyed as long as the Stalker remains alive, the mission continues. The Stalker then proceeds to attacking enemies by itself. Once the Stalker gets downed, he can be revived normally just like a teammate.
 * The Stalker seems to spawn from inside of the cryopod in endless defense missions if the user who he is targeting is somewhat near it or ontop of the pod. On very rare occassions, this may cause a glitch where the stalker will replace the pod health bar in the left of the screen, even with his name "Stalker" replacing the artifact or pod name.

Trivia

 * It seems the Stalker has a chance of spawning anywhere where the player is. At least one boss must be slain before he can target a player.
 * In a team, he will contact only one player, but all players will see the lights flickering before his appearance.
 * The Stalker can also appear in an Endless Defense mission, in the Orokin Void, Nightmare Mode and when you replay the Tutorial.
 * The Stalker is factionless and can be attacked and killed by any other faction such as the Grineer, Corpus and Infested. However, if the Player escapes his grasp, the chance that the other enemies will triumph is very low, as his combat ability is similar to a Tenno. He can also glitch and will not be attacked at all, allowing both the enemies and the stalker to kill you.
 * The Stalker can be harmed by Damaging Abilities, such as Frost's Freeze or Rhino's Stomp.
 * If the Stalker has killed you or is defeated, he won't reappear in the same mission to kill you again, or any other player.
 * Lights will always flicker before the Stalker appears, and right after he leaves/is defeated.
 * The Stalker appears to be wearing a dark-colored Excalibur suit with a unique helmet.
 * The developers will not disclose any information about the Stalker, and act like it doesn't exsist when asked about.
 * As of update 7.11.0, Stalker's loadout was altered to the weapons above. Before this update his arsenal was comprised of a Cronus or a Scindo, and a Braton Vandal.
 * There is a rare bug where the Stalker will be named after the respective boss of the area in his health meter. (i.e Tyl Regor on Titania:Uranus, or Kelade Thaym on Merrow:Sedna.) (Needs Confirmation)
 * There have been occasional encounters with level 1 Stalkers, perhaps due to host lag or glitches.
 * Normally, upon reaching 10% health, the Stalker will announce his failure to defeat you, kneel, and disappear in a puff of smoke. An excessive amount of damage that fully depletes his health before he can do this, however, will cause him to fall over like a downed player. He may glitch, and become invulnerable and frozen after this.
 * It has been noted that Stalker will rarely exhibit much higher intelligence, preforming wall runs, dive rolls, tracking cloaked players, and even actively avoiding enemies and other Tenno. It is unknown if this is due to certain rooms being designed for his AI, or reasons unknown.
 * The Stalker can drop mods and rare resources of current planet when you defeat it.