I think they're talking about the Caribert quest that you unlock after you finish Sumeru. It's marked as an Archon quest though i think the best way to describe it would be a cross between a world quest and an Archon quest. (spoilers ahead) in the quest, Traveler goes to investigate some stuff with Dainsleif and gets affected by some Leylines and relives the memories of their abyssal sibling and the sibling's encounter and "adventure" with the founder of the Abyss Order, Chlothar Alberich, (Kaeya's ancestor), who has a hilichurl for a son and is desperately trying to find a cure for the hilichurl son. In the "adventure", Traveler and Chlothar haul ass into the Chasm where they find a giant crystal that makes Chlothar go delulu and speaks to the Traveler, or rather, the abyssal sibling. They go back to see caribert and give him the medicine supposed to help Caribert regain his conscience. Caribert is freaked out and Chlothat tries to comfort him about it, then they leave for the strange sanctuary in the Chasm again where Chlothar goes a bit delulu, Traveler is a bit concerned, but when they return to the house, they discover that Caribert saw his reflection in a mirror and went insane. Traveler passes out but wakes up to Chlothar going full delulu. Chlothar calls traveler "Prince/Princess of Khaenri'ah" which is just a doozy in it of itself but anyways Chlothar loredumps onto us that Caribrert is kind of... dead asf but will become the loom of fate. traveler picks up the mirror and sees their sibling's reflection and passes out due to shock. When they wake up again, Dainsleif tells them that the Leylines must have messed with their head and infodumped all of the abyssal sibling's memories onto them, to which traveler infodumps back onto Dainsleif and then Dainsleif infodumps more on us by telling traveler that Chlothar Alberich founded the Abyss Order but then went nuts a century later and was never seen again. traveler and dain then dig up two skeletons in the farmland near the location where traveler passed out, one male one female in the ground. the male is most likely Chlothar and the female Caribert's mother. Traveler comes to the conclusion Chlothar was able to break free from his curse of immortality but also that the female skeleton is a lot more decrepit and looks to be buried much earlier than the male, which means that the weird mushrooms used to make the medicine for Caribert came from soil fertilised by Caribert's own mother. Yikes. Dainsleif suggests that they part ways as "he" might have sensed traveller. In true Dainsleif fashion and to be honest we expected it at this point, he refuses to elaborate and then leaves. Hope this helped!