Ya'll, let's not let it get twisted. These two Arks were working together outside of Malekith's sight because for a while there he was quite literally out of sight.
And out of reality, being in the Realm of Chaos.
They were at Albion specifically because they were planning on utilizing the Ogham Circles to supercharge themselves and potentially usurp control of Naggaroth or something equally dire. They are not in Ostland with his approval. Black Arks get a LOT of leeway, often spending not just years but entire decades out at sea, so long as they regularly sent back sufficient tribute to Naggaroth in various forms. That's literally another major part of what the attendant fleets are for. They're not just there to escort the already monolithic Black Arks, but act as their scouts, their flankers, their additional deployers for larger land stuff, their plunderers, their treasure fleets with holds full of loot regardless of what form that loot takes.
He's been working overtime to regain and reorganize his control+eyes+ears across the whole of Druchii-dom, and up until this point hasn't needed to particularly pull on the chains of command over the Claw of Dominion or the Fortress of Eternal Torture. Now, did he have some agents on these Arks at the moment? Undoubtedly. Some of them likely even soul-bound into contracts of loyalty. But up until this point, there wasn't that much of a problem. The masters of these Arks were going on a huge raid, to pillage and raze and burn and punish humans who curtailed their efforts in Albion. Efforts that Malekith likely knows they're lying about every purpose of, but then, the Mists are up and no one can get through. Now all that matters is that the Druchii were, one way or another, effectively defeated in their goals by non-Druchii. That is unacceptable.
A punitive punishment raid on the insignificant humans of the Empire who have never meaningfully (by Druchii-standards) resisted them? Why wouldn't he approve of that? A supposed human 'hero' who has apparently made contact before with the Everqueen herself, and the Asrai, and is connected to another human realm, that of Kislev, and is more than well liked by the High King of the Dwarfs? Sure, absolutely kill him, it would cause ripple damage effects with many of them, or at the very least, cause them pain. Completely something the Witch King would approve. Enough to make him send some of the Black Guard of Naggarond with them?
Why?
What are they going to do, fight back and win? Bitter vengeance and cruelty is the Druchii way that Malekith and Morathi have championed for thousands upon thousands of years, doing their best to grind it into the very marrow and souls of the Druchii all the while.
Besides, the humans don't even know it's coming, the dwarf navy is all the way at Barak Varr, and the Asur fleet is busy near Ulthuan.
And now for a bit of somewhat related rambling.
Canonically, it wasn't until Finubar the Seafarer went ranging around out of Ulthuan and came back that the Asur went 'wait, what? What do you meant the humans of the Empire and elsewhere have gone from mud huts to piles of rocks that are somewhat passable as castles? Cannons, gunpowder, big armies, what?"'. The Druchii never really had that moment, they've been raiding humanity throughout that entire period, and so are/were a bit more appreciable of the changes that humanity went through in terms of advancement. But the point of that is, they still kept winning in their raids. Always raiding, sometimes pushed back, sometimes pushed back hard, but never properly 'beaten' in a fight like they have had with the Asur, the Asrai that one time, the Dawi, Lizardmen, Forces of Chaos, Cathay once or twice, Ind, etc.
But not the Empire, Bretonnia, Tilea/Estalia, not even Kislev. Though the latter I can see being not that worth it given the dearth of coastal settlements and Erengrad being the 2nd or 3rd largest port in the world or whatever, plus all the Ice Witches in the past there being able to sorta freeze the bay which is a whole lot of probably not worth it for the Druchii. Not when Nordland and Ostland and the Westerland are all right there, after all.
So, thinking about how this has gone, retroactively, is sort of a Druchii version of what Finubar went through. Only more violent.
And, yeah, as mentioned, originally, this was going to be a case of 'hey gang, welcome to the turn start, Salkalten is a burning pile of rubble, and there is a huge Druchii army marching its way south into the province, this is how the war turns start, here we go going to have to take war turns just rallying together the Army of the Forest and Army of Ostland into cohesive blocks hopefully before they get too far past the forest line'.
Same way that you were also going to get a thing of 'Middle Mountains gone silent, all settlements lost, all settlers gone, also a dragon died' if you'd sent the Army of the Mountains away.
Circumstances change. In this case, a very lucky, perhaps even potentially divinely blessed given how things turned out, priest of Manann, changed this whole encounter around. Also, yeah, without Sadrina, I get the feeling a lot of players would have assumed the dwarf wall would have held off the Black Ark and stationed their men there and then I would have gotten to write something along the lines of 'watching ten thousand men turn to paste, powder, and reddened mist'.
So. You know.