Deklan knew about this from Adrian.
He found it odd for the Pale Night Lady to be birthing—that many Pale Draggles, and he has his own suspicion about the matter. Initially, he thought it was an innate skill or something, but for an entity to produce that many weaker entities without repercussion sounds broken.
Even so, he kept it to himself.
And as he was forcing them to heed his plan, Adrian confessed out of sheer fear about this.
Both he and his mother were planning to tame the Pale Night Lady by feeding her.
It should be impossible to tame a Ghost, and it was well-established that it was impossible.
Like all Ghosts, the Pale Night Lady came from the Rift of Dusk, not a Spirit—so no matter how much they fed her, she would never be coaxed to assimilate with Adrian. From the start, one wouldn't even become an Exorcist by assimilating with a Spirit.
A Spirit could only make an Exorcist stronger, not create an Exorcist.
