This advanced laparoscopic surgery video presents the diagnosis and management of Accessory Cavitated Uterine Mass (ACUM) in a young woman with severe dysmenorrhea and chronic pelvic pain. The case illustrates how ACUM can closely mimic degenerating fibroids, adenomyosis, broad ligament fibroids, and congenital Müllerian anomalies on routine imaging. Through systematic evaluation with transvaginal ultrasound, colour Doppler, 3D ultrasound, hysteroscopy, and laparoscopy, the correct diagnosis is established. The surgeon then demonstrates complete laparoscopic excision of the accessory cavitated mass using meticulous capsular dissection and fertility-preserving surgical principles. Histopathology confirms the diagnosis by demonstrating an endometrial-lined cavity containing chocolate-coloured fluid. This is an excellent educational resource for gynecologic endoscopic surgeons interested in rare uterine anomalies and advanced laparoscopic pelvic surgery.