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 a uncommon/rare NPC that is always encountered with a level of 30-70 and can be found anywhere, from low level areas to high level areas. He is known to follow anyone, including members of your party.

His presence is made known by taunting the player with messages in the same fashion as bosses. The screen and lights also flicker for the player during these taunts. After three taunts, the stalker teleports into the mission which can be heard with a unique sound effect.

He also uses similar abilities to other warframes and carries the Braton Vandal. This enemy has shields but its amount and regeneration speed is unconfirmed. Speculations say it is a rogue Tenno. However, one of the notifications he gives the player mentions he is there for revenge, which suggests he may be an assassin. Stalker can be killed fairly easily due to a nerf in a recent update. After his HP drops below 10% he sits down and disappears in a cloud of black smoke.

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. 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 or Ash's Smoke Bomb 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 mele 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 defencless to his staggering mele attacks.

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.
 * The Stalker won't appear to warframes of level 5 or lower.
 * In a team, he will contact only one player, but all players will see the lights flickering before his appearance.
 * If he kills the player he came for, that player cannot be revived by other players and must use a revive to continue. The stalker disappears afterwards.
 * 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.
 * The Stalker is also considered the official mascot for the Stalkers Anonymous clan.
 * The stalker is not affected by any Tenno abilities.
 * 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.