Imma be real guys, if the ritual with killing Occtis was intended as a way to make a celestial, I don't think it would have worked. The sun god Tansul specifies that special materials are needed, materials that are dear to him. And of course the heart of his most devoted priest is dear to him. Perhaps Tansul was cruel like some of the other gods and he only cared for his priests in as far as they could serve him, but there is no doubt that the follower he spoke to was devoted. This ritual would work because the priest would give everything for Tansul, and Tansul recognizes that devotion and holds it as something treasured.
But Occtis? Is Occtis devoted to this God in this way? I don't think so. He's not even truly devoted to his family in the way they seem to expect him to be. You bring your BEST to sacrifice, and the Tachonis house certainly does not consider Occtis its best. People make jokes about "oh we don't like this one so he can be the sacrifice lol", but that seems to be the attitude that Tachonis took, and it is not the approach you take when trying to make an CELESTIAL. I think that devotion and willingness are an extremely important part of this ritual, and the Tachonis are ignoring that because they're hungry for power and don't want to "waste any resources", even though that's what you need to do for this because it's supposed to be a SACRIFICE!! You know, giving up something you don't want to?? Because that's what sacrifice is??
Maybe the Tachonis were expecting it to work because they're remembering a younger Occtis. One who still had hope that if he worked hard enough and had enough skill, he could win his families favor. Bases on how Occtis talks about wizardry, I think he hoped that learning to use magic would put him in his families good graces, only to realize that they see him as a pretender even more now. If the Tachonis were hoping to take advantage of Occtis's desire for belonging and love from his family, then they completely missed their window. Occtis hadn't seen his family in years, and he seemed to have no expectation that they'd ever want anything to do with them. He had devoted himself to his own projects and research. He had some angst about his family seeing him as a failure, yes, but he wasn't under some illusion that they'd suddenly start being proud of him. He'd already moved past that, and them deciding to attack and straight up murder him did NOT help their case.
Think about Occtis in comparison to someone like Wickander. If the Halavars decided that their current celestial sucked and they wanted a new one, start-of-the-campaign Wick would have provided the PERFECT heart for them because he was EXTREMELY devoted!! He was lost in that light loving sauce!! If they'd explained it to him all sad he probably would have just agreed to it! And his heart would have made a crazy powerful celestial because again, HE WAS LOST IN THAT SAUCE. In terms of devotion to a higher power, Wick has Occtis beat so bad its embarrassing. He probably still would make a decent sacrifice, because he still believes in the principles he was raised on, even though he's slowly realizing his family is terrible.
The ritual with Occtis was interrupted so of course it didn't work as the Tachonis wanted. But if they'd completed the ritual, I'm pretty sure they wouldn't have gotten what they wanted. There's still power in the Tachonis family and bloodline, so SOMETHING probably would have happened. But the celestial created would probably have been weak, or untethered, or completely mad. Or maybe it wouldn't have worked at all, and the death would have been for nothing.
Regardless, I'm sure Occtis would have found some sardonic humor in how he managed to disappoint his family even as a corpse. (And there's something painfully sad in knowing they would blame Occtis for it failing, while refusing to see that they're the ones who botched it up.)
