The Undertaker
the gravedigger of Graven-Tosk, dead in her own pit
"The Undertaker dug graves and made coffins. She buried corpses and left them to rot. Some refused to do so quietly. Some, she silenced herself. Some died. Some didn't."
The robed woman whose body lies in an open grave outside the stone hut at the heart of Graven-Tosk, a worn shovel beside her. She is the cemetery's apparent caretaker — or its last living caretaker before the The Deadn't killed her. The hut is hers: piles of logs, a dozen barrels, half as many freshly made coffins, and behind it a small garden, now overgrown.
She refused to help Stein, Benzen, and Arga escape. They killed her. She did not stay dead.
After Death
Her corpse fester-hosts "a being both lesser and greater than human, a servant of a faraway underworld." The shovel becomes a fiery polearm of doom; the rune-covered robes swirl around grim feet; the eyes fill with mad purple fire. See The Übertaker for the entity she becomes and the stat block for the encounter.
The Undertaker as she lived was unstatted — she is not a fight, she is a backstory. Anything the party finds in the hut (the coffins, the garden, the barrels) is hers. Some refused to do so quietly. Some, she silenced herself. Whatever she silenced is in the dirt around the hut.
Disambiguation
The Undertaker (this entry) is the body in the grave outside the hut — pre-possession. The Übertaker (creature entry) is the possessed undead boss that rises during the encounter.
The name Übertaker is what the being from a faraway underworld is called once it takes her body. Her own name in life is never given in canon.
Graves Left Wanting, Graven-Tosk, The Undertaker's Hut, The Übertaker, The Deadn't, Stein, Benzen, Arga
Whether the Undertaker was alive when Graven-Tosk was abandoned, or arrived afterward, is unstated. She kept the cemetery functioning — corpses kept finding their way inside, and someone kept burying them — which implies she was the closest thing the place had to a constant. Whether she invited the underworld in, was chosen by it, or was simply the nearest dead body when it arrived is a CANON CHECK. The "ones she silenced herself" suggest at least some of the corpses in The Plague Pit and the Catacomb Corridor are her work.