The gradual change in how it's referred to is so juicy...
And man, with my theory about steles and obelisks using dead languages and blood to command ghosts (specifically devils/the ten billion) to warp time as an alternative to FTL ("less to do with travel than it is with transmission") and the "Tower of Babel" motif that could sort of come with that, I've also recently realized, both stele and obelisk are just more precise words for types of pillars, and what is a pillar but a sort of small tower... Just tying all of the pieces together.
PLUS Alecto asks "where did you put them?" and Varun says "there is a hole at the bottom of their tower", but, well, Teacher said...
“Down there resides the sum of all necromantic transgression,” she said, in the singsong way of a child repeating a poem. “The unperceivable howl of ten thousand million unfed ghosts who will hear each echoed footstep as defilement. They would not even be satisfied if they tore you apart. The space beyond that door is profoundly haunted in ways I cannot say, and by means you won’t understand; and you may die by violence, or you may simply lose your soul.”
And ten thousand million sure is another way of saying ten billion!
So if Varun implies these things are coming from the bottom of The Tower and Teacher says they were put down beneath Canaan, WELL... That math sure checks out, doesn't it!