When the Tarnished first finds Millicent in Elden Ring, she is in a weary state in the Church of the Plague, clutching the wound where her right arm once was. As her adoptive father Gowry explains, she was born from the Swamp of Aeonia and infected with the Scarlet Rot that has come to define that area. That she has lost her arm to the curse is a reminder of the demigod who plagued Caelid with rot: Malenia.
[Warning: The following article contains spoilers for Elden Ring.]
Malenia is an Empyrean demigod and daughter of Marika and Radagon, who are the vassal and Elden Lord in the lore of Elden Ring's setting, the Lands Between. Malenia was cursed with the Scarlet Rot at birth. Similarly, her twin brother Miquella was cursed to be eternally youthful. Nevertheless, the two Empyreans proved to be incredibly powerful and talented. Malenia became an excellent swordswoman and Miquella a genius who worked on curing himself and his sister of their curses. However, before this could ever happen, Malenia battled with Radahn after the Shattering and blossomed into the Scarlet Aeonia - a massive flower that releases spores of Scarlet Rot - to bring her battle with the powerful warrior demigod to a stalemate. This put her into a comatose state while her brother was abducted by Mogh, Lord of Blood.
The blossoming of the Scarlet Rot Aeonia in Elden Ring's Caelid appears to explain what happened to Millicent's arm, as it is likely that Millicent is the direct offspring of Malenia and Malenia alone. The spores released from the Scarlet Aeonia may appear to have purely destructive qualities, but in actuality, Gowry reveals when discussing Malenia's backstory that the Order of Rot represents "The cycle of decay and rebirth." Malenia thus appears to have birthed life out of the decay she brought to Caelid and especially the Swamp of Aeonia, where Millicent and her identical sisters were born and found by Gowry. In turn, Millicent has grown in a similar pattern to Elden Ring's Malenia, Blade of Miquella, losing her arm to the Scarlet Rot that has infected her since birth.
Elden Ring - Millicent Is The Daughter Of Malenia
While progressing through Millicent's questline in Elden Ring, she even admits to believing she is related to the Rot Goddess, though not knowing if she is either a daughter, sister, or "offshoot." Gowry more directly confirms that Millicent is the daughter of Malenia, although he continually refers to her and her sisters as "buds" rather than daughters. Because buds are essentially offshoots of a greater plant, this suggests Millicent and her sisters are akin to budding flowers off of the plant that is the Scarlet Aeonia - that is Malenia.
In turn, Millicent is essentially both a daughter and extension of Malenia. This is why her invader version uses the same Waterfowl Dance that Malenia does and also why she blossoms into a flower of the Scarlet Rot upon being betrayed by Elden Ring's Tarnished protagonist. But more importantly, Millicent's relation to Malenia shows that she lost her arm because she grew in the same pattern as Elden Ring's Goddess of Rot.