"Kind of a long story. So, before the past, like, hell month, or whatever we're in now, the worst thing we'd faced in Sunnydale was this one single really terrible hell day. It turned out in the end that this kid had been beaten into a coma and that his nightmares were manifesting in the town, and that he was also causing everyone else's nightmares to simultaneously manifest in town, which as you can imagine got pretty apocalyptic pretty fast. For a while we were just dealing with random terrible stuff - the school had this random talent show, one of Alex's students committed suicide in the bathroom, his brother had a sore throat and couldn't sing, my sister and nephew were found brutally murdered in their house - but then, then we had to face Alex and Michael's biggest nightmare.
"There are these weird powerful gem things, called the Silmarils. I dunno what all has happened over them, I get the sense that it's a bunch of terrible stuff. They're real, although this one wasn't, this was a magical manifestation that we didn't think to wonder about because we were all sort of operating on dream logic. And Alex and Michael - they're oath-bound to get the Silmarils, and they're oath-bound to kill anyone who withholds a Silmaril from them. I assume that this is the sort of decision that sort of vaguely made sense whenever they made it, but they regret it kind of a lot now and are just stuck with things being the way they are. Anyway, since they're oath-bound, the second the Silmaril appeared they had to drop everything and try to go get it. I didn't know what was happening at the time, they hadn't told me about oaths or about the Silmarils, so I just figured the Silmaril was, y'know, such an apocalyptic-level threat that it didn't make sense to stop and consider doing other things. They kept telling me to focus on ending the nightmares, and then they kept ignoring that to go chase the Silmaril. I can tell you more about the specifics of what happened, but eventually I ended up with the Silmaril - stole it from this terrifying snake demon that lived in a hell dimension, by the way, almost died getting it but then I cut the demon's head off and got the Silmaril and it was honestly super cool - anyway, I had it, and at some point I'd gotten into my head that maybe Alex and Michael weren't actually trustworthy, maybe the reason they were making all these weird dumb tradeoffs to get the Silmaril was that they were actually just evil and only cared about getting their hands on the shiny rock.
"So I hid it. I figured that if they were trustworthy, then they must have been making these decisions because they didn't want anyone else to get ultimate cosmic power, and if I told them it was safe then we could ignore it and focus on ending the nightmares. And if they weren't, well, I didn't want them having a super powerful shiny rock to end the world with. And so they came to me and they asked me where the Silmaril was, and I said it was safe but I wouldn't tell them where it was, and - Alex was so scared, when he tried to explain, as he was trying to give me alternatives, give me any way at all that I could feel safe about things and still let him fulfill the terms of the oath, because if he couldn't think of anything and I said I wouldn't tell him then he'd have to kill me. And I was scared and confused, and couldn't figure out in that moment whether he was trustworthy, and so I said no, and so he shot me and killed me. - I got better, obviously, and then I went and killed him and Michael back, it was this whole thing. Anyway. The point is - Alex would never hurt me if he had a choice about it. But he didn't, not then, because of the oath.
"And then I figured it out, see. The Silmaril - the Silmaril oath, having the world end up in such a way that the oath forces them to kill people, forces them to kill people they love and desperately want to protect - that's their nightmare. The idea that this commitment they made is going to force them to do horrible things again, and there won't be anything they can do about it. And they'd love to get out of it and not have to shoot me if something weird happens and I end up on the wrong side of it, but they can't.
"So that's how I know."