The Angel of Mercy Hospital was a mental hospital located in Manhasset, New York (1 hour west of Southampton) where Emily Thorne's mother, Kara Wallace, was institutionalized. In August 1993, two days after her father was arrested, a dummy investment group bought the hospital and just shut it down, most likely by the Graysons who need a cover up for taking and, presumably, killing Kara Wallace, a potential threat to their plans on framing David Clarke.
Almost 20 years later, Emily Thorne and Nolan Ross visit the defunct hospital to investigate. While walking through the familiar environment, Emily remembers flashes of her childhood spent visiting her mother at the said hospital. They come across Kara Wallace's old room, her bed with bindings, which had the infinity symbol engraved on it probably by either David or Kara, still intact, and some records of Kara's visitors, which included David Clarke and a Charlotte Clarke, who turned out to be Victoria Grayson using David's aunt's name. ("Destiny")