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 90, that will target and attack a single player within the current cell.

This entity will taunt the player with messages, similar to bosses. For the duration of these messages, the targeted player's HUD will flicker, along with any surrounding lights. After three taunts, the Stalker will appear in black smoke 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 Warframe that is capable of using abilities from various Warframes -- Save for his Dispell.

If the targeted player is dead, the Stalker will disappear in a cloud of black smoke. If the Stalker is forced to flee (by damaging him up to 90%), a bounty of blueprints and more can be obtained.

In General
Stalker attacks with high damage output which can kill an inexperienced player with few attacks. The obvious strategy is to keep moving to avoid his arrows and abilities. The Stalker targets only one player at a time, it is a good idea to hide and let your teammate (if any) assault him. If the target player chose to run away, the Stalker will occasionally teleport melee the player, like a Grineer Flameblade.

When in a moderately long range, the Stalker will occasionally use Slash Dash at his target. Avoid getting hit at all cost as it is lethal (around 1,200 damage).

The Stalker is factionless, he will be attacked by all factions regardless of Tenno's prescence. A good idea would be to lure him into a room full of enemies and let them do the job for you.

Shotguns appear to be 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.

Utilizing Warframe Abilities
True to his taunt: "Your TENNO powers are useless", Stalker is immune to all abilities, he will ignore distracting abilities like Decoy and Invisibility, won't be affected by status effect abilities such as Chaos and Molecular Prime, and won't be disoriented by crowd controlling abilities like Pull and Freeze. The Stalker is also capable of Dispell, which purges miscellaneous buffs such as Blessing, Snow Globe, and even Shadows Of The Dead. Only direct damage abilities like Rhino Stomp (minus the suspension) affects the Stalker.

While most abilities are useless on the Stalker, few abilities can still be utilized to assist you.

Mag's Shield Polarize and Trinity's Blessing will charge the squad's shields when in need (and health in Trinity's case), this is a beneficial ability as an unexpected move from Stalker can be lethal.

All buffs provided by the players themselves, such as Rhino's Iron Skin and Roar, and invulnerability of Trinity's Blessing will cause the Stalker to perform a Dispell animation that removes them entirely.

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!

Trivia

 * The Stalker appears to be wearing a dark-colored Excalibur suit with a unique helmet.
 * 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.
 * Upon defeated, Stalker will always drop the Slash Dash mod, unknown if intended or a bug.
 * As of Update 10.5.0, Stalker has a new ability called Dispell.

Bugs

 * On rare occasions, the stalker will 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 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.
 * Rarely, the Stalker can spawn in a defense mission but replace 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 downed teammate.
 * When the Stalker is killed during a mission, the mission complete music is replaced with the Stalker theme. Same thing happens to the defense music, you will continously hear the Stalker music during a wave. After finishing the planet it will return to the normal music.
 * 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 Kela De Thaym on Merrow, Sedna). (Needs Confirmation)
 * There have been occasional encounters with level 1 Stalkers, perhaps due to host/client lag or glitches.
 * If the targetted Tenno is downed already before the Stalker spawned, the Stalker will froze and become invincible.

Media
Stalker's voice used when taunting players.