In the meantime, they can get a head start on English and then phenomenal cosmic power. Over osanwë; they can at least be inaudible in case of eavesdroppers.

Teaching a language over osanwë really does not work very well at all, but in this case it might be worth the tradeoff.

That becomes apparent very quickly.

Immersion plus osanwë translations works much better. But it'd take a long time to become practiced enough to confidently deal with demons; the extra delay isn't that much. Proportionally.

And we are not planning to deal with demons, right?

Depends on if the Thorburns have anything else that can kill a Vala.

I thought there was a thing where they - inevitably made things worse? 

They destroy. That can be mostly aimed but not entirely, and they can make things more like them just by being around them. It's why no one trusts diabolists. Inevitably making things worse is true from the cosmic balance point of view, and almost always ours.

I'm still holding out hope for a non-diabolism 'how to kill a god in three easy steps,' but more likely some of us are going to have to escalate.

Okay. Well, we'll get good at English. 

English: the ancient language of dark powers from beyond the world.

(It's much uglier than Quenya.)

It is! The Noldor politely do not comment! They do spend more time than usual singing!

Even at Noldor rates of learning things, it'll take a while before they can exploit the library very systematically.

Months. Angband is quiet, though.

And more magic is definitely worth a lot of time.

Months is enough time to progress toward other goals. Might want to found a third city here, since it was prophesied the safest place on the continent. (It's also the correct place for Amber to be, since it's where the stolen library is, but that's true whether or not there's a permanent Elf settlement.)

Once people are mostly literate in the new language they can start specializing. Enchantment, elementalism, bogeymen. Much of it looks useful, little of it looks like a promising avenue for killing a Vala. No one touches diabolism.

Nope. There has to be some clever way of combining the other approaches, or something.

It doesn't help that this is a specialist's library and they're avoiding the specialty. They could conceivably weaponize enchantment to detach Angband from everything else and let it sink, but even though the library can tell them how to permanently break connections this is a prohibitively huge scale. Bogeymen might be useful as soldiers, especially since many can be resummoned if killed, but nothing's even close to the right weight class. At least divination helps; they can get vague impressions of Angband continuing to stay quiet.

It'd be too convenient if they could use it to check what happens if they try diabolism.

It would. There are more than zero spirits and Others that can mess with time, but questions about anything too specific and powerful just return a result of the relevant Others themselves being thoroughly messed with. Captured bats resume flying in regular bat patterns, tea leaves just dry out. A real augur might be able to do better but they don't have one of those. No unexpected armies, though, that one seems like a pretty safe guess.

That's something. They make it a site for a third Elf city. They debate whether to get back to annoying Melkor or to focus their attention on more advanced magic.

More magic would be a higher priority, but focusing on that would give away the fact that there is a higher priority. Continue slowing him down by about the same amount as before, and take advantage of the fact that he doesn't know how fast they're adding practitioners?

Unless he flies over and sees how many practitioners there are. ...can he find the city by tracing the attachment lines between people? Ulmo might not have known to guard against that.

...possibly. Disguising connections isn't all that advanced now that they have a library— it'll be a delay to do it for this many people but should probably be a new number one priority. They can just hope he doesn't know already.

They get to work on that. If he knew one assumes the divinations would have been scarier.

Probably. He does have non-army threats that from a bat's-ear view might look like the wrong scale to worry about, so can't be sure.

Disguising the connections is a project, but a quick one. It's probably worth all the practitioners picking up the skill anyway, and this city has comparatively few non-practitioners per capita to cover. Most of the time spent is in the practitioners dropping what they're doing to learn it.

The Feanorians can safely be assumed to notice all of their connections with people in the Nolofinwean host getting altered in this way.

Getting apparently altered; it'd be much less easy and ethical to directly change attachments between people. (Findekáno might have to start worrying about the fact that the enchanters can by now distinguish degrees of relationships by looking. Hiding all connections to outside the city entirely is a fairly convenient norm on that front.)

This one we might want to just tell them. If the Enemy hasn't figured this trick out himself, it can let practitioners be almost as undetectable against his practitioners as against mundane people.