I think this is just Elina's last "fuck you" to Nyx. Nyx was Elina's slave, and she had a shit childhood thanks to her. Nyx pretty much lived like a dog for like 15 years until she found the staff. She pretty much sold her body and soul to the demon to gain power and she finally had a chance of defeating the person who ruined her life. But Elina just defeats her, because Nyx was a dumb whore and she let go of her staff as Elina taunted her that she's useless without the staff. This could be the alternative ending if Elina chose to use the staff. Nyx is betrayed by her former master, the only one she ever trusted or even liked (although he raped her everyday), as her worst enemy teases her. So Elina pretty much stole Nyx's childhood and now even her master. She took everything. Now they are both humiliating and raping Nyx as Nyx now has nothing. It's pretty much Elina and the staff's way of "punishing" her. Nyx was a lowly slave who dared to attack Elina, so now she is being punished. Nyx was a useless, pathetic girl to the staff, so he ditched her and now punishes her as well.