Here's my suggestion for what we exchange with the Eldar:
[] Plan Enlightenment With Eldar
-[] A Warden hero will be on call if the Eldar need them (Provides 1 favor, Provides 1 favor each time they are called in)
-[] The Eldar ask for a series of promotions and demotions among guardsmen officers (Provides 1 favor)
-[] Kesar attends Harlequin dance lessons (Provides 1 favor, locks a social with Guilliman next turn)
-[] The Daemonic Relics in Warden storage (Provides 1 favor)
-[] A library card for 5% of the Black Library (Costs 1 favor, can be repeated four times) x2
-[] Black Covenant Information Probes (Costs 1 favor)
-[] Greater Daemon locations (Costs 0 favors)
-[] A Spar with Khaine (Costs 0 favors, uses an Other action next turn)
-[] Write-in: What is the Emperor of Mankind really planning x2. Depending on what's learned, go with:
--[] The Emperor's Notes (Provides 4 favors)
--[] Eldar lessons on write-in (Favor costs vary): Chaos language fluency.
--[] Write-in: Eldar help with setting up psyker import routes for the Rune World's growth x3
My reasoning for all this, in order: The Warden Hero on call is rather steep looking, and there are many situations in the Maelstrom where we would really want one Hero to deal with a Terrible Horrible Monster threat or just a Really Fortified (because Maelstrom) Militarised World and such.
However, we have a bunch of heroes right now and Kesar is fully back with the Eternal Wardens and has finishes his psychic awakening and no longer has that big malus he had against Epsilon-354 (a minus 30 for his rolls, ouch). Many of the things that the Eldar would want to use an Eternal Warden Hero for are also typically adventures that would only grant them more experience, perhaps even more Daemonsbane tiers such as with the likes of Durante and Vergil (who have their next task be 'recover something from the Warp' which is relatively easy for them even without Eldar help). Or face a Slaaneshi-related threat and such, which is what Wardens are the best at.
The promotions and demotions are, honestly, something I'd be willing to do for free. Basically, as confirmed by DaemonHunter, this is mostly promoting people that are least xenophobic and psyker hating and demoting the most xenophobic and most psyker hating. I'm hyper down for that.
Kesar attending Harlequin dance lessons is great because, as sold to Kesar back on Orus, Harlequin dancing can have
Anti-Chaos properties and they're giving US the favour point for doing this. A social with Guilliman is also fine, especially with another option down below.
The daemon relics are a bunch of Honoured daemons or Honoured-ish daemons that are bound in relics and thus can't escape on their own, and are also too strong and dangerous to break out for an attempt at true-killing them. They have no other real use for us without a lot of time or risk spent, and meanwhile the Eldar want them. Easy sell.
The Emperor's notes. Okay... first off, to smother any worries,
Kesar is confirmed to not be just fully giving away the notes. He'll give away a COPY of the notes, and thus keep the bonus they provide to his research and so.
While the Emperor would be rather 'murderously furious', as I'd put it, if he ever discovered that Kesar gave away these notes, that's also mostly true in general for all this Eldar collaboration going on. Whatever the Eldar would be able to do with this is unknown, and honestly... that's their business. The Emperor is, in my opinion, the type of guy I'm totally down for selling their secrets and knowledge for (to guys that are way, way better than him morally and technologically) in the name of bettering something else.
Like, with civil war on the horizon, we're very likely not siding with the Emperor anyway. Let's be real, if he kills Vulkan and Konrad next turn then Kesar is not gonna take that happily. So, uh... yeah, why not. Also, funnily enough, with it now being put into the Black Library's collection that means other Primarchs (i.e. Magnus the Red) could get access to it too.
Now, onto what we're really getting! That Black Library account. For reference of what just 5% of the Black Library gives, that is the equivalent of the Emperor's Warp notes in terms of knowledge and said notes are fucking gigantic. 5% is mentioned to be able to affect Kesar's Rune research, by
reducing how much years Kesar needs to put into each Rune by 1 to 2 years (which is fucking insane). It's hyper-worth it and there's books for pretty much everything that we could want, including alternate versions of Guilliman's Codex Astartes (including a Chaos version). We want this. It's hyper, super, giga worth it.
Black Covenants are an issue, and one directly connected to each Daemonsbane of the Eternal Wardens as a group made directly to counter them. Doom Slayer was targeted twice by his Covenant, Crescum Auro's was willing to hinder him at Purgatorio and was willing to directly come kill him too if Baldur and a 1000 Wardens weren't there (and if it was just Baldur, then they'd have called Oriacarius's Black Covenant too which means they can TEAM UP). We also get neato stuff if we shank one of 'em!
Greater daemon locations is free, so... why not?
A spar with Khaine! This particular shard is survival focused, and Kesar can match it easily rn. Opportunities like this are super rare, and we did already get this shard summoned so why not. Can't go wrong in learning fighting from the second greatest war god ever to have lived.
Chaos language fluency is useful because, uh, Runes are
also a Warp-based language with conceptual properties that calls upon power and resonates with stuff. Learning the Chaos language can give up to a +20 or so to Rune research rolls, which is a colossal bonus especially for the more difficult Runes.
The Rune world thing is because we're currently at Stage 8 and there are 11 Stages in total:
Stage 8: Properly constructing infrastructure to support expansion and colonization DC95
Stage 9: Expanding to quadruple digits of psykers (rune production ~100% of the Legion's) DC90
Stage 10: Setting up psyker import routes for further growth. No DC
Stage 11: Expanding to five digits of psykers capable of making runes (rune production ~x10 of the Legion's). DC85
The DC is quite high for these stages, even with bonuses and DC reductions, but we roll five times a turn. Yet there's a big problem after we finish Stage 9, in that we won't move further than that this turn.
Don't let 'No DC' fool you for Stage 10. That does not mean we just autopass it, that means we have to put in extra work to get it done. Other actions, of which we only have three of a turn (typically) and can be used for a lot of other extremely useful stuff, would be needed for this. At minimum this would mean that we'd, at a best-case scenario, need to spend three turns to get to this. I would also like to say that half of Kesar's Rune actions are spent for the Rune world.
But with the Eldar, this can be accomplished this time. Stage 11 can potentially be finished THIS TURN. With the Rune world done, Kesar Dorlin will have his full Rune actions available again (10 actions a turn!). The Rune world will also give a boost to Runic research, the Great Crusade as a whole, and decrease the cost of Runes. Not to mention all the Runes that would able to be produced for everyone, which also helps matters! We want this done as soon as possible, and the Eldar can provide.
Finally, what the Emperor of Mankind is planning. We either know or can guess out of universe, because obviously, but in-universe most of the Primarchs are unaware. Most don't even know what a Webway is, let alone why the Emperor would want it. Or why he made the Imperium the way he did, or how disposable everything is, or why he let Lord Commander Militant Eli get away with potentially killing a third of the Imperium and leave the rest in civil war, or why he's doing this Great Crusade or what comes after, or why he's such a fucking asshole to people etc. etc.
This will tell Kesar the full truth, using two favour points. With civil war very possible to happen and looming over a very close horizon, this could be critical information. To possibly prevent civil war, to wage it better if it does happen, to convince others on opposing sides, to deal with the future that will include the Emperor of Mankind. Also, a very synergetic option with a social with Guilliman.
And narratively fitting, after this Epsilon-354 battle.
(This plan leaves us with one favour to the Eldar, but that'll be paid when they next call for a Hero anyways so we'll be even.)
Edit: After some discussion with the Discord server, as yeah just giving the Emperor's notes away is a huge betrayal with all the information and intimate writings of the Emperor's thoughts and past and it doesn't fully fit with Kesar, I've edited the plan to make more sense with what's learned from what the Emperor's plans are and furthering the deal there than just diving in.