Stalker

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

In order to attract the Stalker, a Tenno must have a "Death Mark" upon them. Death Marks are obtained by killing Bosses. Upon receiving a Death Mark, the Tenno will receive an in-game message from the Stalker. After receiving this message, there is a 1.5% + (0.5%*player count) chance that the Stalker will appear in any mission the Tenno embarks on (including Void missions).

When the Stalker appears, he will taunt the targeted Tenno with threatening messages. For the duration of these taunts, the targeted Tenno and their team will experience flickering lights and an unstable HUD. After three taunts, the Stalker will appear in a puff of black smoke, and proceed to attack the targeted player.

The Stalker's arsenal, just like the Tenno, includes a primary weapon, a secondary weapon, and a melee weapon. The Stalker's weapons are a bow known as the Dread, a scythe called Hate, and throwing knives known as Despair. The Stalker's Warframe armor allows him to use abilities from various Warframes; save for his Dispel, which is his signature ability.

If the targeted Tenno is successfully killed, the Stalker will disappear, vanishing into a cloud of black smoke. However, if the Stalker is forced to flee, he will drop a Mod, resource, and, rarely, a blueprint of one his unique weapons. The Stalker will only flee if his health drops below 10%.

Strategy
The Stalker is capable of dealing damage at a rate comparable to a well-equipped Tenno; he can destroy an entire entire squad if they are caught unprepared. It is advisable to warn your fellow Tenno if you have a Death Mark. It is especially important to warn your squad-mates when the Stalker begins to taunt you, so they may prepare for the approaching fight. When the Stalker does appear, you must makes sure to keep moving, in order to dodge his arrows and abilities.

As the Stalker targets only one Tenno at a time, it is advised to hide and let your squad assault him. If the targeted Tenno chooses to run away, the Stalker will occasionally teleport behind them and engage them in melee, much like a Grineer Flameblade. The Stalker also utilizes several other combat techniques, many of which are similar to to the abilities of other Warframes. When at a moderately long range, the Stalker will occasionally Slash Dash towards his target. Avoid this attack at all costs as it bear a potentially lethal damage. The Stalker will also use Pull on a faraway target, pulling its victinity back. This attack penetrates closed doors, as well as cover, floors, and walls.

Several weapons are especially well-adapted to fighting the Stalker. When combating him, shotguns are effective weapon.

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 100% as much damage it absorbed.

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 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.

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
The Stalker's voice used when taunting players. 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 modified helmet.
 * 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.
 * Some people think the Stalker is Tenno. This might be of the Excalibur Warframe and abilities.

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.

Stalker