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 session. 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 arsenal is comprised of a Cronus, a Braton Vandal and an unknown exo-armor that is capable of using power modules from a variety of different Warframes. If the targeted player dies or the Stalker's HP drops below 10%, a cloud of black smoke will appear and envelope the Stalker, who will then vanish.

It is speculated that the Stalker is a rogue Tenno.

Strategy
Stalker attacks with high damage output, he can kill an inexperienced player with a few melee attacks. The obvious strategy is to keep moving to avoid his gunfire. 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 Abilities (even Trinity's Link won't work on him), although Abilities will damage him (except for Banshee's Sound Quake which will just waste 100 energy). 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 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, although more difficult, strategy 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 hp 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 Shuriken
 * Ash's Smoke Bomb
 * Ash's Teleport
 * Excalibur's Slash Dash
 * Rhino's Charge + Volt's Electric Shield while charging.

Trivia

 * It seems the Stalker has a chance of spawning anywhere where the player is. No boss needs to be slain; the Stalker just randomly spawns.
 * He will attack players that are rank 2 and higher.
 * 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.
 * 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 survive is low, as his combat ability is similar to a Tenno's.  He may however, for whatever reason, become neutral to hostiles in the area. This has been confirmed with a level with Grineer where he would walk around with the Grineer without being attacked or vice versa [needs comfirmation on other factions.])
 * The stalker appears immune to most Tenno abilities.
 * Volt's Ultimate appears to damage him.
 * If the Stalker has killed you, he won't reappear in the same mission to kill you again. However. it may reappear to kill your teammate, in the same mission.
 * It is speculated that the Stalker is of Orokin origin.
 * Lights will often flicker before the Stalker appears.
 * 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.
 * The Stalker has his own battle music, and is the only enemy in the game who does.

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