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 killed or 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. If he disappears without leaving a body after defeating him once, it means that he's not dead and may attack you again later in some other mission. 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 Moult. 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 escape from an otherwise fatal Melee Combo.

However, it is possible to use Loki's Invisibility, Ash's Smoke Bomb 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.

Abilities
It uses already existing abilities from other warframes:
 * Ash's Teleport
 * Ash's Shuriken
 * Ash's Smoke Bomb
 * Excalibur's Slash Dash
 * Rhino's Rhino Charge

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) 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!

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.
 * Bringing a low ranked equipment or using a low ranked frame may increase the Stalker's appearing chance. (Needs confirmation)
 * 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, he won't reappear in the same mission to kill you again, or any other player.
 * It is speculated that the Stalker is of Orokin origin.
 * 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.
 * 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.
 * Players should be very mindful of his Slash Dash, as with his new melee scythe, he can cripple, and possibly even outright kill an experienced Tenno.
 * He can also appear in the Orokin Void.
 * The Stalker is able to appear in the Clan Dojo, but will not be able to kill you due to weapons and abilities being disabled unless in a duel. (Needs Confirmation)

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