The Cold, Calculating Jack

"I know of only one other castoff still in the city - the albino assassin Matkina. They call her the White Death. She wore a camouflaging cloak and brought destruction in her wake."

 is a main quest in Torment: Tides of Numenera.

Synopsis
Apparently, a castoff helped build the resonance chamber. Casmeen and Mimeon don't know the name of this mysterious artificer, but the other castoffs might. Only one of my 'siblings' is presently in the city - an albino assassin named Matkina. Perhaps she can give me answers.

Walkthrough

 * Finding Matkina is a bit of a challenge, as everyone knows here - yet very few know how to find her. After exhausting leads on the surface, head to the Underbelly and talk to Mapper. He will exchange Matkina's lair for the location of the Changing God's own underground refuge.
 * Talk to Ch'kekt. If you resolved the standoff with Peliai (Shaky Foundations), he will take you to the lair. However, you can also as the Master Foreman to construct you a digging machine out of the corpse of the dead stichus buried in the tunnel near the exit to Cliff's edge.
 * However you arrive there, you will arrive in the Sticha Lair. Make your way to the exit to the Buried Crossroads, then return to Mapper and relay your tale. He will point you to the Caravanserai and from there to the Cave of Last Words. To easiest way to access it is to use the digging automaton you can ask the Master Foreman to create.
 * Appearing in the Cave leads to Matkina understandably freaking out. As she decides to forgo disemboweling you, you learn that she is working on unlocking a Merecaster that would allow her to understand and recover her stolen memories. She asks you to enter the Merecaster in her stead. Agree.

Mere

 * This opens a completely new interface. You witness the world through the body of another Castoff, Tash. With Anamnesis you can easily recall your memories and remember that, as Tash, you're a scout, and Matkina is your spotter. You serve a militia under the command of Paj Rekken. Both of you are specialists, assassins, but in the village you are present to promise something to the villagers and procure a jahk.
 * Wait for the ritual to complete. Kina will lead you to meet Nang, in the Kholn Village. As it turns out, the Sand Knights want to wipe the village out with the sanctions of the settlements of the Waste. You can use (Intimidation) or  (Persuasion) to convince Nang.
 * As it turns out, he's more resilient. You can use (Persuasion) to bring him in line (you can safely ask him about what you don't understand and whether he has been outside the Waste).
 * You continue to the sealed-away cave. You can use (Anamnesis) to dredge up a memory, otherwise, continue onwards. You will come to the jahk, kept aloft by an unknown mechanism. You can use  (Quick Fingers) to snatch it or  (Lore: Machinery, Quick Fingers) to disarm the trap first.
 * Paj confronts you outside and traps Matkina. Turns out, the entire point of finding the jahk wasn't to get the villagers to move, but to lure the Sand Knights into a confrontation - and by destroying the jahk, unleashing the teratomorphs on them. The villagers slaughter the Sand Knights, shifting the balance of power in the Waste, and now you decide Matkina's fate.
 * You can agree to help Paj wipe Matkina's memories or use (Deception) either to falsely wipe her memories or have Paj throw Matkina off the cliff. Of course, you can always refuse and be fileted by her men (as you will be if you fail a deceptive check).
 * Afterwards, you can choose to follow Paj onto the ship or stay behind to help the village. You can also use (Persuasion) to have her make a token attempt to save some of the villagers.

Back home

 * Once you arrive, Matkina will react to the choices you make. The canon version of events will be altered by the choices you've made, by altering reality through the merecaster. She will point you towards the Valley of Dead Heroes and Miel Avest, the castoff sanctuary, and provide you with a code (3-4-3-1) to access it.
 * On to finding the builder!