Stalker

The Stalker is an ominous, vengeful figure that infrequently appears during missions, encountered with a level dependent on the current mission, that will target and attack a single player within the current cell.

In order to attract the Stalker one needs have a "Death Mark" obtained by killing a boss. Upon receiving a Death Mark player will receive an in-game message from Stalker. After this there will be a 1.5% + (0.5%*player count) chance that Stalker will appear in any mission (including Void missions). This entity will taunt the targeted player with messages, similar to bosses. For the duration of these messages, the targeted player and team'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 Dispel.

If the targeted player is killed, the Stalker will disappear in a cloud of black smoke. However, if the Stalker is forced to flee (by bringing him down to 10% hitpoints), he will drop a Mod, resource, and rarely a blueprint of one his unique weapons.

In General
The Stalker has a high damage output which can wipe an entire team out if they are unprepared. It may be a good idea to tell teammates that you are Death Marked and that the Stalker is taunting you so they can prepare for the fight. When he does show up, 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(s) (if any) assault him. If the target player chose to run away, the Stalker will occasionally teleport and melee the player, like a Grineer Flameblade.

When at a moderately long range, the Stalker will occasionally Slash Dash toward his target. Avoid getting hit at all costs as this can be lethal (around 1,200 damage).

The Stalker will also use Pull if you stray too far, pulling you back to his location regardless of where you are. This includes closed and locked doors as well as all cover, floors, and walls.

The Stalker is faction-less, and so he will be attacked by all factions regardless of Tenno(s) presence.

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

More exotic weapons, on the other hand, will cut the Stalker down to size in mere seconds, properly modded. The Ogris is capable of eradicating him via a pair of well-placed shots when properly modded, especially when buffed with Magnetic damage. Combining the Synapse with Volt's Electric Shield will also let you shave off most of his HP and shield bar extremely quickly. In either case, though, the player must be ready to escape should he teleport, Pull, or Slash Dash.

In addition, for an extra touch of irony, the Stalker is no more immune to the insanely high headshot damage of his own bow than you are, and his Throwing Knives will also prove effective.

Utilizing Warframe Abilities
True to his taunt, "Your TENNO powers are useless", the Stalker will ignore all support abilities (including Sentinels'), such as Snow Globe (as of Update 10) and Chaos, and is able to purge buffs, such as Iron Skin and Blessing with his ability, Dispel. Offensive abilities will damage the Stalker, but will not apply additional effects (e.g., Rhino Stomp will damage, but not suspend him).

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 the Stalker can be lethal.

When low on health, the Stalker will sometimes cast an ability which resembles Nyx's Absorb barring its visual effects. Attackers within the blast range will receive damage.

Other Resolutions
If the victims wish to keep themselves from harm, they can avoid the Stalker when he spawns by maintaining a long distance about 50 meters or more apart between him, as the 50 meters is considered to be the ability limit for all Warframes, this includes the Stalker. However, the Stalker is still capable of teleporting to you.

Victims can escape the Stalker's attacks entirely by completing missions accordingly and quickly, in order to minimize the chance of an intrusion by the Stalker.

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
 * It has been noted that the Stalker does, though rarely, exhibit much higher intelligence, performing 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.
 * As of Update 10.5.0, the Stalker has a new ability called Dispel.
 * The Stalker will toss his Despair in the air when in an unalerted status (usually caused by a bug).
 * If ever the Stalker appears and disappears in one of the victim's missions, his Combat Music will replace the conventional Combat Music for the rest of the mission.
 * The Stalker seems to spawn from inside of the cryopod in defense missions if the character he is targeting is somewhat near it or on top of the pod.

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.
 * 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 attack 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 continuously 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 targeted Tenno is downed already before the Stalker spawned, the Stalker will freeze and become invincible.
 * The Stalker can appear in the Tutorial (if played again by more veteran players)
 * The Stalker can appear in the Mastery Rank tests.
 * Stalker will become permanently invulnerable after using his Absorb ability.
 * The Stalker can kill you while you are at the continue screen at the defense missions
 * Sometimes, the Stalker may say that the player has killed a boss other then the most recent one killed.(ex.The Stalker may state that the player has killed Lech Kril, despite the player not even having Kril's system unlocked)
 * The Stalker will sometimes taunt and attack players below Warframe rank 5.

Media
The Stalker's voice used when taunting players.