After the Wedding That Never Was, Strahd von Zarovich became a Darklord, and turned the Zarovich Valley into his domain of Barovia. Though they had been transported to the Shadowfell, the Ladies Three positioned themselves to remain neutral, as they always had. The Weaver was diplomatic in her relations with Strahd, even after the Desecration Of The Mountain Fane and killing of her sister The Huntress, and he largely paid her and her covens no mind.
However, after the Count caused the Flooding Of Berez, the swollen Luna River engulfed many of her followers. The Lady of the Swamp Cursed Strahd to never find happiness in an incarnation of Tatyana Federovna, and for this transgression, he hunted her. She attempted to avoid his wrath, but after a few years, the Vampire lord eventually the Weaver in the Ethereal Border, hiding at the standing stones where her fane was once worshipped.
Empowered by unnatural strength granted by Vampyr, Strahd skewered her, and her blood spilled in the Ghostly realm. Using knowledge of cursed magic that he gained from the Amber Temple, he performed a ritual that left her fane untethered in the Ethereal; using The Weaver's own blood for the rite, he completed the Desecration. With the Lady's control severed, Strahd subsumed her powers over Barovia's waters without resistance.
Strahd had already desecrated the mountain fane, and would later complete the Desecration Of The Forest Fane, gaining total control over shaping the landscape of Barovia. He would maintain this ability until the Slaying Of Strahd.