Have to kill the Elves have to kill the Elves if there's no way to kill the Elves at least have to hate the Elves, and they do hate them, and they're terrified. 

"I might need to know more about how this oath stuff works in principle."

"...when we give our word it binds us,"  Fëanáro says. "If they've sworn to not do something they can't do it, if they've sworn to do something they can delay but at great cost and eventually they end up doing it."

"Right now they can't kill you, and they're dwelling on it but it doesn't seem to be directly harming them..."

"If you cannot make progress towards a goal you've sworn to achieve, the oath is at rest."

"I might be able to do that. I might mangle something horribly the first few times I try but keeping them around alive is... not really viable is it."

"No. You think you can - render them incapable of harming Elves?"

"Maybe."

"Are they in pain, were we right about that?"

"If they are they aren't thinking about it right now, too scared."

"Okay. And we can't very well make them less scared...Do you want to try it?"

"It is not going to be the most fun I have ever had but I don't have anything lined up that I'd rather do."

And she looks at the orc and looks harder for what exactly the orc has sworn -

To be an orc, and pursue orc greatness, and hate Elves and kill Elves and bring Elves to Melkor, to serve and obey Melkor.

Well, she's got nothing if not license to experiment, here.

She will leave the first two bits alone.

Killing Elves or bringing them anywhere (because presumably Melkor can move around and cause various locations to be where Elves are to be brought), nope, can't do it - can't even leave a self-defense loophole there this thing is fucking insidious - hating them, no, no opinions south of, let's say vague annoyance, about Elves as a species - she will have to see if this compels them to find reasons to individually hate every Elf they run into or if it'll just work - annnnd - unable to hold any actionably confident opinion on the opinions or orders of Melkor even if he is standing right there saying them over and over again?

There's a moment of silence.

The orc is terrified, but just terrified. It doesn't hate them and it isn't going to kill them - arrange their deaths? it can arrange their deaths - but how -

Nope you may not do that either no.

Now just terrified.

"I think this one is de-oathed now but still very scared."

"Well, if it can't hurt us -"

It can hurt them, just non-lethally! It has lots of ideas from Angband on how to do this.

"Hurting you sans lethality is not in the oath so I didn't cut it out how crippled do you want it -"

"Can't do more than return any injury dealt to them, and only that in self-defense?"

"We'll see if that just leaves it inclined to provoke you until you stab it or something but okay -"

Prod.

"Elves are not very likely to respond to provocation with stabbing, particularly stabbing someone magically constrained from hurting them. Though I guess that doesn't have to be likely to be a way the oath ends up channeled."

The orc is not brainstorming provocations. If it did get stabbed, though, it'd be bound to stab back.

"Well, it will definitely stab you if you stab it, but it's not presently thinking about ways to get you to stab it."

"Great," he says. "That - seems good enough. No?"