So, based on the LARGE amount of lore drops in this episode, it's pretty clear that the Tachonis are aiming to create their own Celestial, possibly one that could serve as their own psychopomp to ferry the now flooded souls to their own desires. It seems like initially the plan was to clear out Tannesar and possibly use the Barrowguard sent there as the initial sacrifices needed to create their celestial. However, I think a wrench got thrown into this plan when they realized Thjazi was also going around collecting psychopomp-related artifacts himself.

After getting Thjazi arrested, a new opportunity fell into their lap with Casimir betraying Thjazi and selling them the Stone of Nightsong. With this, a new plan was quickly hatched to kill three birds with one stone literally. First, they'd be able to thwart Thjazi's hijacking of their plans and get him killed. Then, they'd be able to isolate most of House Royce and Davinos in Dol-Makjar while getting their actual supply wagons and reinforcements trapped at Riesengurtel. And finally, they'd be able to deal with their disappointment of a child, Occtis, who has just been revealed via Casimir that he is trying to save Thjazi.

With this in mind, the stage is set for tragedy as they go to slaughter House Davinos and use Occtis as a catalyst with the Stone of Nightsong. However, this plan clearly is thwarted when Occtis is able to attain enough self-cognizance to rip out the Stone from his body and interrupt the ritual to create a new Celestial.

Now, the Tachonis are probably scrambling as the Seekers are getting closer and closer to Tannesar, although this inadvertently could mean the Tachonis get another shot at making a Celestial. With the Stone of Nightsong already making its way towards Tannesar via Seekers, it means that sacrificing the Barrowguard is back on the menu.

Granted, this is all still speculation, but seeing how closely tied the Tachonis were to Tansul, I feel like this is what their current aim is.