In campaign he was a Tomb King High Priest and his Dragon model was akin to a black hole.

Chill king has priest in army? That is really weird.
Do you use KMM? If not please do, base game mod manager is pretty unstable.

Does this happen even if you have just required mods on? Unlocker, vfx and this mod?

Maybe mod file is corrupted try redownloading it (unsubscribe, resubscribe).

I tried KMM and it broke the game, and needed to be re-installed. I think I could just wait till later.

Really? Sorry to hear that, Kaedrin Mod Manager is kinda essential for most mods as base mod manager is unpredictable at best.

Installing KMM should be relatively easy, I am sure there are decent guides for it but in short

1. disable all mods in normal launcher
2. download and unpack KMM somewhere, it should not matter where
3. run it (either let it detect where is the game or maybe you will have to show it in settings)
4. give it some time to chew trough all mods (first start takes little longer few mins if you have lot of mods, later starts are faster)
5. try to run it with just KMM and no mods on to check if everything is ok

KMM has lot of advantages like easy control, profiles, sorting and lot of other things, but main one is that mods with it are predictable.

That Lich there looks like something weird is happening with starting script and broken textures on dragon are just weird, texture linking is relatively straight forward thing I have no idea how something like that can break just for some users, as that is something that either works or doesnt work for everyone (unless you have some very unlucky mod conflict like mod that has same named textures.).

Of course its up to you but I recommend KMM either way.

In co-op, my friend was playing this faction, and whenever he'd try to use any of the ability spells, it would desync us and boot us. Is this a known thing or it's just not liking co-op?

That is not known, it should work it co-op, but admittedly I havent tested co-op much, so will do it sometime probably today to be sure.

Do you both use KMM, have you tried it with just required mods, do you both have required mods?

I havent encountered such issue, but I imagine maybe if one of you is missing vfx submod then something like crash on cast could happen.

Does it also happen in custom MP battle, or just campaign battle?
Did you tried all types of spells? Mana costing ones, army spells, LL abilities?
In single it works for both?

If all fails, try redownloading mod, maybe one of you has damaged pack.

I think there should be a factionwide leadership penalty for peasants and normal knights. I think a -5 or -10 could work for this. When first starting up a campaign, it is said that the peasants aren't happy with this (something along those lines). I wouldn't be happy either if I was moved north by a giant ice dragon to a frozen island covered in fog then sent fighting norscans and druchii. Bretonnian knights, while they do like their quests, probably don't wanna serve under a dragon, something they aim to kill for their grail quests.
From a gameplay point of view, emphasizes the use of snow constructs which are arguably worse than peasants at fighting. There are also a lot of leadership buffs you can get anyway to circumvent this.

I think chaotic wasteland should be moved to unpleasant instead of habitable. Sure the Chill King and his Unseelie knights might be doing fine there, just some chaos stuff going on but again, the peasants. There is nothing to grow there, horrific abominations, mammoths, norscan raids, and so forth make it hard to live there for peasants.
The unpleasant climate makes it possible to remain there, just a bit harder to do so. It also presents a hard choice to make. Occupy Norsca and eventually get money and etc from it or razing it to boost chivalry for all the buffs. Bretonnia and the Empire are very close to you anyway, you can occupy them instead for the pleasant climate and bigger settlements.

The unique skill "Treasury supervision" should be buffed to reduce -10% upkeep and -20% recruit cost in own territory. The control, global recruit, and leadership is just too good to give up.

The unique skill "Quality armor" should have +10 armor instead of +8.

My brain says "greater arcane conduit is super strong for a spell lore that's also strong + flying + physically strong + dragon" but my heart says "mhmmm dragon..." Make of that as you will.

I don't think he should have vows because I don't think he would really care that much about ze lady.

Overall loving the mod, I got a 17% wardsave and a +25 winds of magic pool items within 10 turns so woo.

Yea I agree, peasants and knights unwilling to work for Chill king should be reflected somehow, I already did part of it with starting army, which has non of those in it. I also plan to reduce peasant economy "cap", as for knights, will think about something. I will also probably give some faction debuff that will be removed when you liberate Albion from chaos hands, similar to Karak Eight Peaks for dwarves...

As for usability of snow constructs, also connected to "chaotic wasteland", I plan to have some buffs for ice units on snowy areas like Norcan peninsula, something like regeneration, ward save, vigor, that sort of thing, things you would expect from snow constructs when they are on the snow and problem with "chaotic wasteland" is that those are basically snow planes so I am looking more on snow aspect and less on chaos aspect in this, I want also those areas to be source of Eternal Ice resource.

All this will be combined with ability to spread "snow", sort of ice corruption, that from certain threshold will generate Eternal Ice and gives those buffs to local armies and at same time reduces effectivity of peasant structures like farms as it is hard to grow crops when everything is frozen.

So it will basically be, going north? Use constructs. Going south? Use peasants. Want Eternal Ice? Go north. Want Eternal ice and constructs south? Spread winter/corruption there.

As for balancing of those skills, yea I will recheck it. Defensive and Crusading trees should be more balanced.

And to vows, well I want this to go in sort of King Arthur sort of way, with Chill king being basically knight king.....dragon....sort of.....that is why I am also considering giving Lore of Ice only to some sorceresses, so I think vows should be there, maybe bit modified. One of which could be vow to Unseelie court that will raise cap for Unseelie units, so each decently leveled general (like 15lvl) will be able to increase cap, as generals will need to earn "respect" of Unseelie court to have those units.

All in all campaign side of things need some work, which is why it will be main theme of next update.

Btw thanks for feedback.