"...maybe all serial killers are actually insane demon hybrids."

"That would be sort of a nice idea because it would mean I can stop thinking how weirdly horrible people can be? But somehow I doubt it."

"It would not be nice to think I was doomed to become a serial killer!"

"… You said all serial killers, not all demon hybrids, and besides, you prefaced that with 'insane'?"

"The insane part was me thinking that all demon hybrids eventually go murderspree because of evil magic reasons."

"Oh right. Well. Hopefully not, and that would not be super reassuring if it were the case, no."

"Anyway, probably won't happen, and if it does we have all these helpful non-hybrids around to stop us, I don't think either you or I could do anything about Eva if she decided we were a threat. ...well maybe I could with some magic but then Willow could help."

"Yeah." Shrug. "Hopefully she doesn't turn out to be secretly evil."

"...who, Willow?"

"No, Eva, the one who can probably take us all down."

"Oh, right. An evil Slayer, yikes."

"Yeah."

"Anyway, any other experiment ideas?"

"Oh, uh, I don't think we checked my shoulder?" he says, then pulls up his sleeve.

It's gone by now.

"Should maybe repeat that to get proper results, but it seems like it still changes things if they're not visible. Uh, is there a way to do something to me that's noticeable to you but not to me? It might only reverse it if I know about it, or it might only do it if I can specifically object to it?"

"Sure, I'm gonna do a thing and not tell you about it." More Latin, magical gestures, and now Theo has a blue spot in the middle of his forehead and a green one on his back.

Presumably the gestures didn't indicate the points, so Theo will stand there, blink, and say, "And presumably you have some way of getting feedback on whether that worked and I didn't just totally nullify it because for some reason my demon magic might work that way?"

"I have some way of getting feedback, yes—oh, we should maybe compare to, I'll turn your hand blue how about that?" And he chants again.

Nod.

"So we can compare the progress of this one with the others, is what I mean."

"Yeah," he agrees.

And is his hand turned blue?

It did indeed! Now they watch.

The blue spot in the middle of his forehead disappears more quickly than the blue on his hand, though they both disappear relatively quickly.

"Take off your shirt."

He raises an eyebrow but does so. "I'm assuming you put something on me somewhere, not just asking me to strip for your own purposes?"

He doesn't answer but walks around Theo to look at his back.