The Stalker is a mysterious figure who hunts down Tenno accused of serious crimes. When the Tenno kills planetary bosses in Assassination missions, there is a chance that they will draw the Stalker's ire. If so, they will receive an inbox message (with the sender masked as "???"), labelled with the name of the character they killed to trigger the mark.
Your actions have consequences….
From this point on, a Tenno so marked will have a small chance to encounter the Stalker during their normal missions. The Stalker will announce his presence with three taunts, before initiating a lockdown of the map and appearing before the Tenno. He is armed with powerful weapons and abilities akin to those of a warframe. In addition, he has the ability to nullify any warframe abilities that are used against him. If he kills his target, he will leave victorious, but if his health is brought low, he will attempt to flee.
The Stalker appears in the special alert A Favour for Darvo. He has a major antagonistic role in the Second Dream quest. After the quest is completed, he will appear as the stronger Shadow Stalker, but his dialogue will remain unchanged. He is later mentioned in Operation: Shadow Debt, which features his Acolytes, mysterious characters that follow his ideals, though the Stalker himself does not appear. The Stalker appears again in the New War quest, still allied with Hunhow, although without his Sentient armour and weaponry.
"I know your every move, [name]."
"[name], there is no place to hide."
"[name], you can't run from your past."
"The murder of [boss] will not go unpunished."
"There is no salvation for your crime against [boss]."
"The blood of [boss] is on your hands. Did you really think there would be no repercussions?"
"I am your reckoning!"
"Your sentence is death!"
"You shall not leave this place!"
When a warframe ability is used against him
"Your Tenno powers are useless!" [casts Dispel]
Upon defeating the Tenno
"It is done. [name] is no more."
"[name], you shall not trouble us again."
"Justice is served. [name] has been executed."
Upon being defeated
"What have you… done?"
"No?! This is not… possible."
"I have failed. [name] lives."
"I… have failed… this one will remain… unpunished…."