Here's an absolute big swing guess for next week's episode, involving Tachonis' plans in Tanisar and why things went down the way they did at the Palazzo (with several other sub-guesses on new lore from episode 17).
My big swing is that Julien is intended by the Tachonis to become the vessel for/physical aspect of the Deva Vindicta, or that he's at least an option they're considering, and that one of the ways that he's being "prepared" to serve this purpose is by inducing a great desire for vengeance in him. I honestly don't think I can possibly be right, but I've got enough red string that I want to talk about it, because it's a fun idea.
We've never found out why the two primary targets of the attack on the Palazzo seemed to be 1) Occtis, who's been explained, and 2) Raimond Davinos for some reason? Maybe it's a simple as that he's a powerful warrior and they wanted him off the table quickly, but I have a hunch there's more to it. They were sort of vague about wanting to kill Aranessa, and Primus fully walked away from Julien. "Well struck, warrior," he said before leaving. Maybe killing Raimond and the house of Davinos in general will serve the magical purpose of pushing Julien into deep vindictiveness and desire for revenge, which will be necessary for the Deva.
Furthermore, we learned in this episode that the people who were sent to the Barrowdell by the twins were specifically Julien's battalion, specifically his students. Why was that choice made? Did they specifically request his battalion? Is it just because he was gone, or did the Tachonis choose his people to send, knowing it would fuel his desire for revenge and draw him after them to Tanisar, where maybe they need him to be for the ritual?
The Tachonis seem to need to do some sort of procedure on someone, in Tanisar, in connection with the creation of the Deva, and we have multiple references to an "apotheon." There's a "Chamber of the Apotheon Vindicta," guarded by the "Hall of the Demon Guardians," and a long bridge over a lake that goes to the "Apotheotic Obelisk." An apotheon is a mortal who is deified or otherwise elevated to godhood/adjacent - so I'm guessing from these words that the Tachonis need not only the sacrifice of Occtis, but a person to be exalted into this deva.
"Our most recent findings were from about two-and-a-half months ago. The final experiment was a failure, and wasted our last supplies of ossement."
"A new source of calcidian (?) has revealed itself, but we need to move quickly. The Cloak is moving faster than anticipated. If the obelisk holds fast in Tanisar, does the sacrifice itself need to happen in Tanisar?"
My guess is that "ossement" is the (un)dead equivalent of the Halovar's filament, which is harvested from a living celestial. The Tachonis, maybe, are making ossement by refining artifacts/remains from dead celestials, hence their interest in the stone and the coffin? Maybe "calcidian" is the unrefined version, or otherwise somehow used to make the "calcidial prosthesis"? These are all bones words, so I'm guessing they're connected, and the -ment suffix connects it with filament for me. The obelisk is the Apotheotic Obelisk, which connects the concept of an apotheon to the deva ritual.
"The Halovars have proven a dead end. Some inherent power of their sorcery immunizes them against the procedure."
I read this as evidence that filament and ossement are inverses of each other. Putting ossement into a Halovar, or otherwise doing the procedure, doesn't work because they're immune due to the filament - which seems different from what happened to Tertia.
"Final experiment a partial success, but largely failure. It is my opinion that Tertia's innate gifts and connection to the Tenebral Reaches thwarted the grafting of the calcidial prosthesis. I will miss her dearly."
So Halovars don't work at all, and it's the opinion of this lord that the fact of Tertia being a sorceress at all made the experiment fail.
Which brings me back around to Julien: if they need a person to serve as the apotheon for the creation of the Deva Vindicta, and they need them to not be a sorcerer, and they need them to be swamped with a burning desire for vengeance, who's a better choice? They seem to have tried a Halovar before they tried a Tachonis, so they'd prefer someone from outside their circles - i.e., a Davinos is better than a Seramai. Julien is potentially the most well-known knight of any vassal house, given his work in the Rebellion, and is publicly known to be something adjacent to vengeful in nature, given his relationship to Thjazi.
I know this is a huge swing, and I think it's honestly too complicated and centers Julien too much to be the real plot, but it's fun to think about. It would also give Julien a deeper tie in to the lore questions, if he becomes some sort of partial-celestial OR if the procedure goes sideways because he's cursed, and it would give him reason to reevaluate his relationship with vengeance if it's also tied to Tachonis plotting.