What emphasize the theme of the mystery of death in Hamlet. In the play within a play.

The Mystery of Death

In the aftermath of his father’s murder, Hamlet is obsessed with the idea of death, and over the course of the play he considers death from a great many perspectives. He ponders both the spiritual aftermath of death, embodied in the ghost, and the physical remainders of the dead, such as by Yorick’s skull and the decaying corpses in the cemetery. Throughout, the idea of death is closely tied to the themes of spirituality, truth, and uncertainty in that death may bring the answers to Hamlet’s deepest questions, ending once and for all the problem of trying to determine truth in an ambiguous world. And, since death is both the cause and the consequence of revenge, it is intimately tied to the theme of revenge and justice—Claudius’s murder of King Hamlet initiates Hamlet’s quest for revenge, and Claudius’s death is the end of that quest.

The question of his own death plagues Hamlet as well, as he repeatedly contemplates whether or not suicide is a morally legitimate action in an unbearably painful world. Hamlet’s grief and misery is such that he frequently longs for death to end his suffering, but he fears that if he commits suicide, he will be consigned to eternal suffering in hell because of the Christian religion’s prohibition of suicide. In his famous “To be or not to be” soliloquy (III.i), Hamlet philosophically concludes that no one would choose to endure the pain of life if he or she were not afraid of what will come after death, and that it is this fear which causes complex moral considerations to interfere with the capacity for action.

Another excellent articla on the theme of death, "Death in 'Hamlet," can be linked below;