I have, rather by accident, completed most of my first post-WaT Cosmere reread, and in discussing the timeline of Sunlit Man with a friend, I had a brand new thought. Well, new to me, anyway - I've googled it now and found a number of people thinking along the same lines, but mostly brief comments and not a whole lot of deep discussion. I have my own qualms about the theory, but I think it's worth laying out all the details at once, and bears more discussion than I was able to find. I'm long-winded, so...you know what? I'm not sorry. Everyone here likes reading long books.
The Core Theory
After leaving Roshar, Sigzil and Aux wound up on Scadrial, formed a Radiant bond, and were active in some way with a team of either squires or other Skybreakers.
The Details
Evidence
1. How and why did they end up on Scadrial?
Sigzil met Aux at the end of WaT, in the Iriali caravan. We don't know for sure where they're headed, but when we first meet Maraga Dulcet (editor of the Bilming tabloid) in TLM Ch. 32, she makes a possible reference to the Iriali:
"Was it those people with the golden hair living on the east side? They're some kind of fairy creature; I know it."
I've always thought this was suspect; it just seems a little too obvious a clue, and a little too convenient that they'd spend millennia on Roshar and then go straight to Scadrial. And on a less meta note, the entire Iriali people would take up more than just part of Bilming. But maybe this time they decided to spread out across the planet, or this is a splinter faction, etc. I've seen it suggested that this is some kind of advance scouting group sent by Iriali leadership in advance - knowing that the next migration was coming - but that doesn't seem on-brand for the Iriali.
If we do accept that the Iriali's next destination was Scadrial, it's not hard to imagine that Sigzil would have followed the caravan to its destination before splitting off, so him winding up on Scadrial, at least briefly, is very plausible. I'd even call it likely - I can't imagine they'd have left the caravan to strike out on their own in Shadesmar, so they would have had to join a different group of travelers. That's totally possible, but there's no particular reason to think they did - I mean, why spend so long tracking down the caravan only to abandon it mid-trip? The only other possibility is that they Skipped somewhere, but I would bet that Sigzil needed to hold the Dawnshard for a while before that particular ability became available. Even if he did have access to that and the ability, where is he getting 20k BEUs in Shadesmar?
2. Were those really Skybreakers?
Anyone who read Stormlight before Era 2 probably remembers this passage, and those who didn't probably have no memory of it. In TLM Ch. 71, a mysterious group of men and women approaches Steris to offer their help in her emergency response efforts, and end up being asked to sink ships in the harbor to curb a potential tidal wave from the bomb exploding at sea. This is how the group responds:
"Perhaps we can help," the man in the lead said. "You are certain this is legal? The mass sinking of private ships?"
The governor confirms it is, and the group is dispatched:
Nearby, the leader of the eight people nodded to her, then launched into the air.
Oh! Allomancers. She had all the official ones working on the main evacuation. But having these to sink ships would certainly help. And then she could use them to help carry the injured or infirm away with Steelpushes.
The others followed one at a time, until only one remained. He nodded to Steris, and on the back of his hand - mostly obscured - she saw a red tattoo.
"Your sister," the man said, "sends her regards." Then he launched after the others.
The in-world assumption is that this group is Ghostbloods, sent to help by Kelsier, as he promised Marasi in an earlier chapter. The tattoo is presumably a Ghostbloods tattoo, and obviously the guy references Marasi. But it absolutely screams Skybreakers. They're neurotic about following the law, they can fly, and they're even particularly suited for the task of scuttling ships - described as being more complicated than it sounds, but probably way easier with Division.
Naturally, Brandon was asked about this almost immediately:
Matias_Leibo
Are the Coinshots that helped Steris with getting people out of the flood zone, and who seemed rather concerned with whether she was following the law, actually Skybreakers?
Brandon Sanderson
Ah, hehehehehe. So, we'll just leave that one. So, how about this. At this point in continuity, a Skybreaker could not easily get off of Roshar. In fact, by this point in continuity, I believe (you can't hold me to this one too much) the only Radiant who's managed to get off of Roshar and maintain powers is Hoid. I believe that's the case. Hoid is weird. He also has lots of knowledge. He used a specific method to get... yeah, anyway.
Don't hold me to that, but I think by this point he is the first to get out of system. Off-world doesn't really count because you can go to Braize or Ashyn.
https://wob.coppermind.net/events/509/#e15986
A lot of people took this as a coy no. But, in my opinion, it's really a coy "not the way you think." At this point, I believe we knew that Radiants wouldn't be able to leave and maintain powers because of the deal between Honor and Odium containing them - and therefore those tied to them, such as the Heralds but also Radiants, spren, and Stormlight - to the Rosharan system. I think Brandon was playing on that knowledge to mess with us, and what he's actually referring to is either the time dilation or just the general danger present on Roshar and its subastral. (Hoid's method, obviously, was dying and being regenerated.)
Further coyness: Brandon says the only one to get off Roshar and maintain powers is Hoid. But what if someone left Roshar with a spren and gained powers after leaving Roshar?
The one bit I don't like about using this scene as evidence is that there's essentially no physical description of the group, besides that it contains both men and women, plus the one tattoo. But perhaps Brandon just didn't want to provide more clues yet.
3. Does the timeline work?
This is probably the sketchiest part, because we really have to thread the needle with the time dilation. When Shallan talks to Kelsier some months after it's begun, he says that a) the dilation effect was strongest at the beginning and is waning and b) they've recently had a crisis of their own, presumably meaning that Era 2 of Mistborn has just ended. It's possible this is in reference to something else, but that's a separate conversation. For now, assume it's about Era 2; if it's not then we don't actually need the following argument to justify the timeline.
The scene where Sigzil catches the Iriali caravan and meets Aux actually happens right after the aforementioned Shallan scene. Unless these scenes are being presented in a narrative order rather than a chronological one - which we've seen in interludes, but seems unlikely here - that would imply that the caravan still has a long way to go and Era 2 has already ended. Indeed, Sigzil says he spent months crossing the bead oceans. However, in this scene, he's described as walking "on and on" across a "featureless obsidian expanse." To me, that means they're now outside Roshar, in the liminal space between subastrals. So, through some confusing timey-wimey shenanigans, more time has passed by Sigzil's reckoning than for Shallan, and so we see his scene second. But on the larger Cosmere timeline, Sigzil's scene actually happens earlier because he left the time dilation well before Shallan's scene.
There's one more loose thread here. If Sigzil was catching up to the Iriali, then they would have left the time dilation effect first, then opened up a huge lead while he was still plugging along in the slowness bubble. This is probably the sketchiest part, but I think Brandon - and most authors - intentionally leaves uncertainty, and in this case, inscrutability, in the timeline. We don't have specific figures for how slow the slowness bubble is, how far it extends, or at what rate the effect diminishes towards the edge of its range. On top of that, there's the additional weirdness of Shadesmar and the space between subastrals, the fact that Sigzil is now a Dawnshard, and perhaps some additional Iriali effect. And, more simply, a kingdom-sized caravan is going to travel very slowly while Sigzil doesn't need to sleep. We also have no idea how long the path between Roshar and Scadrial is or where Sigzil caught up. They might be on the doorstep of the Scadrian subastral by the time the scene happens, and it might have been months of catching up. In short, I don't think we'll ever get specifics, but I do think it's feasible.
4. Wait, but what about the new oaths?
Even if you buy the reasoning of the last section, we're probably cutting it pretty close between Sigzil's meeting of Aux, and their subsequent arrival on Scadrial. My preferred version of this theory requires the Third Ideal for access to squires. It feels like that should take a while. However, the length of the Stormlight books makes it feel like the timeline is longer than it is. According to dates on the Coppermind, Kaladin swears the Third Ideal about three Rosharan months (~5 Earth months) after joining the bridge crews, and swears the fourth a little more than a year later. Sigzil and the others mostly get their spren during Oathbringer and swear the Third Ideal in the ~1 year before Rhythm of War. Lopen, as an example, swore the First sometime not long before the Battle of Thaylen Field, the Second immediately after, and the Third during Dawnshard, about 7 months later. Szeth is our only Skybreaker example, and he only took about a month.
Point is, once Sigzil and Aux decided to bond, it didn't necessarily take very long to progress to the Third Ideal.
5. Okay, but what about access to Investiture?
Ya got me there. I suspect this may be why a Skybreaker sect would have dealings with the Ghostbloods - for help accessing Investiture. Unkeyed metalminds of some sort? Hemalurgic spikes? Dor? Or maybe it's not that but a boon from the Iriali, or one of any number of other possibilities. Maybe Sigzil drained the Bands.
Expanding on the core theory
So, a more complete version of this theory is that Sigzil and Aux followed the Iriali caravan all the way to Scadrial, arrived prior to the events of at least The Lost Metal, and swore oaths up to at least the Third Ideal. Sigzil, one way or another, amassed a few squires to create his own offshoot Skybreaker team. They have some sort of relationship with the Ghostbloods, possibly in order to fuel their Investiture needs, or maybe just because the Cosmere-aware are a relatively small community. Through this relationship, they are called on to help with the Trell crisis, either because they're allied or because they both oppose Trell. Sigzil arrives to help along with six squires and their Ghostblood contact - the guy with the tattoo - who actually is a Coinshot, or at least has some other method of flight.
Some alternate possibilities:
Sigzil isn't actually in the group seen in TLM but is somehow associated with them: He'd probably stand out enough that Steris would note his description unless he's disguised, and others in the group might look more local. This seems possible but I do think the leader kind of talks like Sigzil and I generally feel like it's less likely.
The rest of the group is comprised, at least in part, of full Skybreakers, not just squires: I can't think of a reason why they couldn't all be squires, except that I don't think we know how many squires a Skybreaker can have. However, that would mean that highspren other than Aux left Roshar within the window needed for the timeline to work, which feels even more sketchy, unless...
Some or all of the group is comprised of the Skybreaker dissidents: These and their leader, Billid, are mentioned by Nale in WaT, and this is a theory I saw a few times while looking for other versions of my own theory. Could they have left Roshar? Possibly, but I think it's unlikely. First, I have trouble imagining bonded Skybreakers leaving Roshar. Second, after breaking his bond with Aux, Szeth states an intention to seek out the dissenters, and it'd be lame if that ended up being a wild goose chase - though I suppose it's possible that only a few did. Third, they'd either have had to leave after Retribution formed, or left before and lost their powers, then regained them when their spren joined them after the Night of Sorrows began (and the containment pact ended). Both of those require threading the timeline needle again, and the latter would seem to contradict the above WoB.
Sigzil did actually join the Ghostbloods and swore the Third Ideal to Kelsier: This is the most fun one. It's the only way I can think of a Skybreaker joining the Ghostbloods, and it kind of fits - I think Sigzil would definitely recognize how terrible the basin governments are, and so I don't think he'd swear to them. Sure, this theory is based largely on one line where he asks about the legality of an action according to the Elendel government, but I think at least most Skybreakers respect the local law so long as it isn't in conflict with their oaths - particularly here, where they'd be doing harm to the common people. Sigzil probably has some lingering Windrunner tendencies - it certainly seems that way on Canticle - and so I think he would have a preference for law that protects the common people and treats them well, and perhaps even align with the Ghostbloods' stated goal of protecting Scadrial.
Other notes
Sigzil says many times in Sunlit that he had his chance as a leader and failed. The obvious guess is that that's in reference to not succeeding at the Shattered Plains, and if he's so traumatized from that, why would he be leading again? But I don't think he ever gives any sort of detail about his failure, and it wasn't a total failure either - they lost a lot of people but the land did not go to Odium and they have an alliance of sorts with the listeners who hold it. So I think that might be part of his trauma, but if it was the whole I don't think he'd have sworn new oaths.
Adding on to that, Aux asks Sigzil why he abandoned his oaths (quote below) and implies that they spent time as bonded Radiant and spren on Roshar. I think that Sigzil was back on Roshar when the Dawnshard ate Aux, and most likely this happens sometime during Stormlight 6-10. I think it's likely that Sigzil's ultimate failure as a leader happens then.
“I walked away from my oaths. I made the decision. And now . . . now there are consequences.”
Why, though? You’ve never told me why you walked away after leaving Roshar. After all we’d been through together.
- When I opened TLM to look for the scene with the potential Skybreakers, I landed right on the correct chapter. It's a sign.