Nature has been so very careful to maintain the proper order (such as it is) that Rachel knows this must be an aberration, a mistake, accidentally overlooked by the powers that be. Because if they knew, there's no way they would let something like this, some pleasure like this, exist.
Ivy's tongue on Rachel's clit is a miracle, a revelation, an abomination. Perfect and mind-blowing and nothing Rachel has ever experienced could have prepared her for it.
Usually, these things, pleasures like this (though there is, in fact, no pleasure like this of which Rachel is aware) are reserved for those pairings that make evolutionary sense. The elation of a power-pull with another witch is one example, and there are others. They keep things in order, preserving balance through the eons.
But this, ohgodthis, this is nothing that should ever have been allowed. The pleasure of Ivy's teeth in her neck is a paper cut compared to this, this, this, which is something much closer to an opened jugular. This is the pleasure of pain, of death, of conquering death, of being born again. It is the hot white heat of a star being birthed somewhere in the cosmos, the explosion of a supernova inside Rachel's skull.
Somewhere, distantly, she hears her own voice saying or screaming: "This must be wrong," but Ivy seems to know Rachel isn't asking her to stop. Maybe she understands, maybe she knew this was going to happen, maybe this is the great secret that vampires protect in order to survive.
Maybe not. But there’s something about Ivy's touch that makes Rachel feel as if Ivy knows her, as if Ivy knows every muscle and sinew and imperfection, that she can sense Rachel's body down to the very cells and maybe deeper.
And then, and then it happens. Even with the greatest pleasure, even when things are so fucking right that Rachel can't imagine they could improve, things change. Ivy's bloodlust can be held at bay for only so long and Rachel's skin is singing at Ivy's touch, at the sharp scent of Ivy's arousal and anticipation of the moment when Ivy's teeth will break the surface. For the moment when there will no longer be a Rachel and an Ivy but merely predator and prey (though, with Ivy, it is never entirely clear who is preying on whom, and that is the greatest mystery of their push-pull).
And then. Rachel feels the scrape of Ivy's teeth along the skin where Rachel's leg meets her body. She can feel her pulse in the artery there, as if the blood itself is crying out for Ivy's touch. Maybe Ivy can hear it, too. Her teeth break the surface and—
Rachel feels the line. She didn't ask for this, didn't seek out a ley line, didn't open herself to it or offer it, but somehow, somehow, Ivy is pulling it through her. The bright electricity of power surges through her, straight from her head to her heart to her clit and she wonders if this is what dying feels like. She tastes something like burnt sugar, feels the burning thrum across her sweaty skin, feels-
Oh. The circuit completes. There is the line, there is Rachel, there is Ivy. Rachel becomes a conduit and a resistor, feeding power through to Ivy through the place where her teeth meet Rachel's vein. Ivy gasps or screams or begs for death or forgiveness, for this to end and for it never to end.
There is something between them more intense than an orgasm, more shocking and more primal. Later, Rachel will remember this feeling, remember how it fails entirely to fit in the order of the universe, of evolution as she knows it. She will wonder if this, the screamshock of pleasure that Ivy drags through her and pushes into her, is some part of evolution's great march forward. If they are on the cusp of something.
For now, though, she touches Ivy's hair and all Ivy can do is laugh.