They expect to experience when the king arrives in the room. This is likely a religious belief that God takes the spirit of the dead with him.
Explanation:
The poem, "I heard a fly buzz- when I died," by Emily Dickinson tries to capture the thoughts of the dead in the event of his death. He imagines a crowd of mourners by his bedside and the stillness that accompanied such moments.
He and the mourners with him expect to experience the last Onset when the King arrives. Given the widely held belief in those days, this is likely a reference to the immortality of the soul where God takes the dead along with him.
