Murder most foul, as in the best it is, but this most foul, strange, and unnatural.
Alone with Hamlet, the Ghost reveals the truth: he was not bitten by a serpent but murdered by his brother Claudius, who poured poison into his ear as he slept in the garden. He commands Hamlet to avenge him but spare Gertrude.
The Ghost reveals the central crime — Claudius murdered his own brother by pouring poison in his ear.