Answer:
The mystery of death.
The play Hamlet's major theme is death. It is the death of the King Hamlet that triggers the events in the play one after another. When the Prince Hamlet hears about the news of his father's death, he comes back to Denmark.
Explanation:
Source:
https://www.bachelorandmaster.com/globaldrama/hamlet-themes.html#.YN-eHuhKiCo
