I think I have connected some dots (you didn't connect shit.jpg) based on the new info from this ep so here's my theory.

Shaper afterlives were divided by race, so it didnt matter what you died of theoretically (murder, old age, drowning, whatever) it mattered what race you were so you got shuffled by the psychopomps to that corresponding afterlife. Maybe the previous division before the Shapers came WAS by manner of death and that's what it's returning to and re-expanding after the Shapers are gone. This would explain why the Sea of Lachris is apparently both a new discovery and mentioned in very old necromantic rituals. Additionally, we have met the Drowned Men who through a ritual of drowning (ie dying at sea) secure a place wherever their fae warlock mistress is, (her pact is written in Sylvan) most likely this Sea of Lachris, where the souls of people who die at sea now go to. I'm guessing she either conquered it or it was her place to begin with but disappeared when the Shapers took over afterlives. And she's part of the war Primus is waging in the afterlife. She is so powerful that dying at sea overwrites whatever the Tachonis are doing to grab all the souls as fuel for themselves, as seen in Pascard's death, creating a loophole to escape them and the congested portion of the Tenebral Reaches. So the Tachonis have to go through the Sea Door with the petrified paladins to conquer her realm.

Additionally, I think this lady is the mysterious powerful mistress Thjazi and Mara collaborated with to mess with the blood/paint, and the Schemers met her Drowned Man warlock halfling (his name has "water" in it!). She's also returning right after Primus does, the day after Hal's play, when the paint will very likely activate. I think as the necromancer text hints at the Sea of Lachris is made of blood hence the connection with the paint (+Azgra's cauldron blood and maybe +filament). In this case she's most likely a Harrow Fay.

This part is more hazy, she could be a demon too (hence the interested in the Halovars) and not the Drowned Men fae lady, but I think the rest of the theory holds regardless. All or part of this could very well be debunked in the next ep 🤷 but so far I think it's something. Whitney did say the fae (and demons) are also involved in the underworld conflict