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Navani strikes a tuning fork beside an infused diamond. When she pulls the fork away, a thin thread of Stormlight follows it; when she touches the fork to a second, empty diamond, the Light streams into the second stone — and continues to flow for as long as the fork keeps the new vessel vibrating. She times the flow on her control run and notes it on her pad. Sometimes she thinks of Stormlight as a gas, she writes; sometimes as a liquid; sometimes she suspects it is neither, with a little of both.

For the real experiment she works inside a steel box her scholars Soulcast for dangerous tests, a fat glass window cut into one end, the top left open so any blast will vent upward and her low vantage will be shielded by the wall. She has made the singer guards drag the box in from the hall and set it on her desk for her — she has told them it is just routine precaution, and she has been careful not to act as though she expects an explosion. She does not, in fact, expect one. The sphere that killed her scholars on the day of the invasion was not Voidlight; it was something stranger. She is now convinced that mixing Voidlight and Stormlight will produce not destruction but a new Light, the way Towerlight is a third thing.

She runs the experiment as before — drawing Stormlight out of one diamond, sending it toward another — and reaches into the box with tongs to set a Voidlight diamond in the middle of the stream. The Stormlight does not react; it simply parts around the dark stone and continues to the receiver. When her fork falls silent, the suspended thread puffs away into the air. She had not expected anything, but now for the harder test. She has been working for several days on a single hypothesis: if Stormlight answers to a tone, then Voidlight and Towerlight will answer to tones too. She has had to teach herself music theory on the side.

Alethi music uses a ten-note scale arranged as two five-note quintaves; the Thaylens prefer twelve. Anciently, she has discovered, there was a three-note scale, and a handful of compositions survive in it. The first of its three notes is the very pitch that draws Stormlight. To get tuning forks for the other two she had to send Fused through the Oathgate to raid the music conservatory at Kholinar — and to her delight, Voidlight answers to the third of the three notes. She has been unable to find any older record that knew these three notes once mapped to the three gods, and no Alethi scholar in her reading has suggested the first tone makes Stormlight do anything in particular; Raboniel, asked, confirmed she knew it already and seemed mildly surprised that Navani has only just stumbled on what the Fused call the pure tones of Roshar. Navani has tried singing them; she cannot hold the pitch well enough, but Raboniel can, and can move a stream of Light from one stone to another by touching, then walking, her finger from gem to gem while she sustains the note. Today Raboniel is out, so Navani uses the forks instead. Three pitches. A note for Honor, a note for Cultivation, a note for Odium. Vorinism, of course, worships only Honor. Theology will keep.

She sets her next experiment. She draws Stormlight and Voidlight as twin streams from corner diamonds and crosses them at the centre of the box. They push on each other, swirl past, and continue on their separate ways to their separate forks. She tries a different angle. She picks up the partly-emptied Voidlight diamond and a freshly-filled Stormlight one; the principle of fabrial design is that infused stones are vessels, and that a stone with even a little Light drawn out of it becomes a pressure differential that will pull in a spren or more Light. She wants to refill the Voidlight diamond, now partly emptied, with Stormlight. She strikes the fork, starts the Stormlight streaming, and tries to make it pour into the Voidlight stone using the matching tone. The instant the fork touches the Voidlight stone, the tone dies as if she had doused a candle. She can bunch the Stormlight up against the dark gem; she cannot get it to enter. Even when she draws the Voidlight steadily out to the side to maintain a differential, the diamond refuses Stormlight. Only when the Voidlight is entirely gone will the stone accept Stormlight at all. *Like oil and water indeed,* she mutters, writing it down — though the fact that the streams pass through each other without violence also tells her the two are not opposites.

She crosses the room, casual, and lets her hand drift to the vein of crystal in the back wall where the Sibling lives. The guard sees only that she is browsing the back shelf again; she does this often. She asks the Sibling whether anyone is watching. The Sibling reminds her, with familiar irritation, that Voidspren cannot become invisible inside the tower — that protection is separate from the one that suppresses enemy Surgebinders and Raboniel has not yet corrupted it — and that there are none near. They press her back: they do not like the work she is doing with fabrials. The man who forges weapons may claim he never killed anyone, but he is still preparing for the slaughter. Navani answers that she does not see another path to restoring their Light, and steers the conversation back to Towerlight. The Sibling complains that this is growing tedious. *Do you want to be saved, or not?* Fine. Towerlight is the Sibling's Light, the Light they could create on their own, or have their Bondsmith create through them; it powered not only the tower's defences but everything else, and the defences fail now because the Sibling, robbed of the ability to hear two of the pure tones of Roshar, can only produce a trickle. Navani latches on. Two tones, not three? No, two. One from each parent. The note of Odium is an interloper, false. She asks whether the Sibling's recent collapse could be tied to Odium becoming a real third pure tone. The Sibling admits they do not know. She writes the hypothesis down. The Sibling pushes again for what really matters: find a way to restore Towerlight and purge their system of the Voidlight in it. Navani says quietly that this is exactly what she is working on. She needs an emulsifier of some kind, something that can stick to Stormlight and bind it to Voidlight.

She breaks off — she has been at the wall too long. She picks a book at random and drifts back to her desk. There she finds a small box waiting for her; the door guard nods that yes, Raboniel sent it. Inside, a brightly-glowing diamond. At first glance it looks like Stormlight; held next to a true Stormlight sphere it shows the green tint. Lifelight. Raboniel had promised to obtain her some, and someone, Navani suspects, is connected to the Sibling losing track of Lift. With anticipationspren popping into the air around her she runs her central experiment with the new Light. The middle tone draws Lifelight just as she had hoped — she can pull it out of one diamond and send it streaming into another. Towerlight is Lifelight plus Stormlight, after all; Lifelight, the Light of Cultivation, might be the binder. Holding her breath she runs every experiment with Stormlight and Lifelight she has been saving up. Nothing. She tries Voidlight and Lifelight; she tries all three; she runs through the brainstorm list and runs it again and again — each pass burning a little more Lifelight off into the air — until the diamond is empty and she has nothing to show for any of it. Shooing exhaustionspren away, she stands and paces. Earlier she had been unable to draw Towerlight from its own diamond at all, even with two tuning forks at once. Another whole morning, lost.

She knows she should not let it bother her. Real scientists understand that runs like these are not failures; they are the necessary first dead ends. The trouble is that scientists do not usually work on deadlines like hers. Every day a new clock to outrun. Her only real lead is making the Lights mix — that or, perhaps, that strange air-warping sphere that killed her two scholars. Whoever first masters this, she suspects, wins the war. She also realises that if she does discover something, Raboniel will know within hours — there is always a guard watching — and any answer she finds will be Raboniel's too. She needs a second plan. If she does find a way to make Towerlight, and the shield does fall, she will need a way to keep the crystal pillar isolated long enough to act on it. So she walks the stacks pretending to read titles, and sketches, in her pad propped open in her safehand, a notion: she has been told she can have anything she needs for her experiments, and she may store equipment outside in the hallway. What if she builds disguised fabrial weapons — painrials, heating fabrials — and stores them, looking like failed prototypes, in crates against the corridor wall? She could even arm them off Voidspren gems she can demand for research. It quiets her to have a tangible plan, but the puzzle itself still itches. What is Raboniel really after — a weapon herself, like the thing that destroyed her scholars?

After a respectable interval she goes back to the stacks, picks a book, sits in the chair she has tucked there, and pretends to read while she touches the vein. Any spren near? No, the Sibling sighs. She asks about the explosion on the day of the invasion, and about a Light that warps the air around it like heat. The Sibling felt the blast but cannot identify the cause; they have never heard of any such Light, and it sounds dangerous. She asks about emulsifiers and how Towerlight is mixed from Stormlight and Lifelight. The Sibling corrects her with what is, for them, near gentleness: Towerlight is not mixed. The two Lights come together, as one. *I am the product of my mother and father. Towerlight is the product of me.* Stop asking the same questions in different ways. They have told her what they know. She asks whether they have managed to eavesdrop on Raboniel — they have not, much. They can only listen near a few people of relevance: the Windrunner; the Edgedancer, who they think has been surrounded by ralkalest and so is invisible to them; the crystal pillar, where they hear only echoes through the field; and, curiously, one Regal who speaks all languages and sometimes vanishes from sight entirely. Of Raboniel's experiments they can only report that she is trying her own Light mixing and getting nowhere — that she has not, the Sibling notes with a flicker of grim satisfaction, gotten as far as Navani has. *That is doing something for my self-esteem,* Navani thinks. Perhaps fabrials made of a Stormlight-Voidlight hybrid would work in the tower even when the defences turned against her; perhaps that is what Raboniel wants. The Sibling shoots her down. You cannot outthink a Fused thousands of years old. *You'd better hope I can,* Navani says.

She tries a new angle. What if they found the Sibling a candidate to bond? The Sibling rules out humans entirely. She asks, gently, about a singer instead — there is a singer she trusts, Bridge Four's Parshendi, whose people rejected the Fused long ago. Could the Sibling theoretically bond him? Silence. *Sibling?* They had not considered the possibility, they admit. A singer who does not serve Odium. They will think on it. It would, at least, surprise Raboniel, who believes the Sibling dead or asleep. But not now — the Sibling cannot form a bond while the protections are still up. They would need him to touch their pillar. *If I had him here, ready, with distractions in place to give you time to talk to him —* The Sibling reminds her gently that they cannot bond just anyone; in the past they spent years winnowing candidates from Bondsmith squires, and even those, in the end, betrayed them, though less badly than other humans. Until the pillar's powers are restored, no bond will produce Towerlight either, so the chosen candidate would not yet be useful. *Better focus on that problem first.*

Fine, Navani says, and sees her opening. She needs the pressure of imminent collapse off her, or she cannot work; she needs the nodes defended. Tell her where one of them is. She has plans — a list of measures she can read off. Silence on the other side; she keeps going. Send Kaladin loud and obvious on the wrong floor as a decoy, then quietly bring up troops and a load of unset crem to seal the node's chamber entirely — perhaps even sheathe the crem with training-blade sheaths for Shardblades, to give defenders hours. Or: if she knows where a node is, she can have Kaladin start infusing it with Stormlight to fight back the Voidlight that has been pushed into the Sibling. Raboniel corrupted them through a node; perhaps the Sibling can be cleansed through one. Worth trying, since her Light-mixing is currently stuck. She waits, gripping her pad. *Humans are like persuasionspren,* the Sibling says at last. *I can't speak with one of you without being changed.* She waits more, because silence is best here. *Fine.* One of the two remaining nodes sits in the well at the centre of the Breakaway market — clever, since the well is full of pump fabrials the modern tower's scholars do not entirely understand and an extra gemstone in the cluster will not be obvious. On the first floor? In one of the busiest rooms in the tower? Yes. All the nodes are on the lower floors. Her last Bondsmith had planned more, higher; only the four on the first few levels were ever built before the project ran out of resources. This particular node, the Sibling adds, can be reached from the back ways through the aquifers — let the Windrunner try, both to destroy the threat and to infuse the node with Stormlight as a counter-corruption test. *And the fourth node?* That, the Sibling says, is theirs alone. Prove the experiment with this one and they will speak further. Navani thanks them, leaves the wall, fetches a stack of books for cover, and walks back toward her desk meaning to study Kalami's old notes on gemstones that use buzzing vibrations as substitute letters — only to look up and see, on the back wall, the Sibling's light flashing far too brightly. She hurries over and presses her hand to the wall, asking what is wrong.

*They've found the node in the well. We're too late.* What? Already? *I am as good as dead.* Contact Kaladin. They already have it and he is too far away. *Contact Kaladin. Now. I'll find a way to distract Raboniel.*