Donkerling & Eva Dewulf "Theory"
Alright, this is not really fleshed out, but I think there are some connections here I want to pick other people's minds on. The tl;dr version is this: I think Eva Dewulf very likely drowned herself and her two grandchildren as part of some sort of ritual/sacrifice to "Donkerling" (or really, the Blackbone), and it seems plausible that this sacrifice has some relation to the original "deal" Hendrik and Henry VIII made about Caer Ys.
Lets start at the start: Hendrik—presumably at the time he was granted Brancrug or very soon after—met in secret with Henry VIII, who pressed him three times to break some sort of promise that Hendrik had made in relation to Caer Ys. This tells us that Hendrik probably had some sort of connection (close enough to make "a promise") to Caer Ys before becoming Baron Brancrug. This could have come from his short-lived time as a "priest" or his mercenary company's alleged role patrolling the Bounds of the Mansus or something else. In any case, it seems pretty likely that this connection to Ys is why Old Coppernose gave Brancrug (which seems to have something like a "portal-ish" relation to Ys) to a former Dutch mercenary with seemingly no connections to the English court in the first place.
The conversation also suggests that Henry VIII had something specific in mind that he wanted the Dewulfs to do about Caer Ys. For now I'm agnostic about what exactly that was.
Sometime after, Ernestine Peterhans, Hendrik's right-hand woman, becomes familiar with a trapped Long in the Stairs Tenebrous called "Donkerling." Donkerling seems to be a servant of the Blackbone, and Ernestine starts sacrificing mice to it in exchange for information about shipwrecks. Of course, what Donkerling really wants are sacrifices of "new-drowned child[ren]" but Ernestine draws the line at mice.
Interestingly, reading that same book ("The Shadow in the Stair") suggests "there are hints that [Peterhans] may have been planning a salvage expedition." I don't think that just means she was planning a standard salvage expedition to ordinary shipwrecks Donkerling told her about (is that really the kind of thing you only "hint" at? And couldn't a reader already imply that from the earlier part of the text discussing shipwreck news?) I think this means something more ambitious: an attempt to salvage something from Ys, probably related to the Henry/Hendrik deal.
What is Caer Ys? One of the things it seems to be is deep underwater (most of the time). And which hour holds influence over "the crushing black of the deep sea"? The Blackbone (probably). So if you needed to get to Caer Ys, it seems likely that some sort of ritual to the Blackbone or sacrifice to one of his servants might be your best ticket.
We are not given any suggestion that Peterhans or Hendrik ever succeeded in making a voyage to Ys, although on my reading of "The Shadow in the Stair," they were probably trying. And the next few generations of Dewulfs seem more interested in their various esoteric hobbies than in Caer Ys. So if the Dewulfs had made a promise to Henry, they do not seem to have fulfilled it, which may relate to the several "curses" swirling around the family by the time Eva comes around.
Eva of course, picks back up the relationship to the sea and to Ys. Connie has a few lines emphasizing that something in Eva's lineage made her important to Ys. Eva's own writing focuses on her concern with incurable "faults," likely referring to her own inherited curses. The theme of "the Crossing to Noon" seems to be Eva and Bancroft trying to escape Eva's various curses to reach Port Noon across the Sea.
Then of course, Eva drowns herself and her two grandchildren. Unfortunately, we really don't have any details about this (except that their bodies were never found), but it seems striking that the Dewulfs were given Brancrug as part of a deal with Old Coppernose to do something about Caer Ys, and that the (legitimate) Dewulf line ended in a sacrifice that looks a lot like the sacrifice we are told that the entity likely controlling access to Ys desires. Am I onto something here? Or has the fascination taken hold?
A few obvious follow up questions: What did the Dewulfs actually promise to do about Ys? Were the five generations between Hendrik and Eva doing anything about it? And what were the actual consequences of Eva's last ritual?
An alternative Occams-razory reading of these facts might also suggest that Donkerling just killed Eva and the kids, but that seems less interesting, and its not clear how you'd reconcile it with Sauri's insistence that Eva drowning herself wasn't the full story (besides Sauri just being wrong). Another possibility might be that Eva drowning her two grandchildren was the price she paid the Blackbone for her own safe voyage across the sea to Port Noon.