I know no one cares about Naberius Tern but that is exactly the problem.
Every single lyctor we have seen so far is undone by killing their caviller. Mercymorn makes it very, very clear that she has been carrying the grief of killing Cristabel for ten thousand years. Palamedes doesn’t say he would rather die than kill Cam to become a lyctor because he is busy dying rather than killing Cam to become a lyctor. The unspoken rule is that lyctorhood can only be obtained by killing the person most important to you.
Unless you are Ianthe.
Ianthe is the only lyctor who isn’t in a constant state of mourning. She has achieved immortality and cosmic power without losing the person closest to her.
Incidentally the person Ianthe loves the most is Corona, who also doesn’t care what happened to Babs. Ianthe chats with Corona while wearing Babs discarded corpse and they banter about it.
Ianthe essentially made her Faustian bargain with counterfeit money and I think eventually she is going to have to pay up