While the people of Denmark have been told that their king died when a venomous snake bit him as he slept in the orchard, Hamlet discovers that the truth might be something else entirely. The King's ghost informs Hamlet that this account is "a forged process of my death." Instead, the ghost tells the prince that he died when Clausius poured a strong poison into his ear.
As the ghost says, this "leprous distilment" quickly moved through his veins and curdled his blood as if it were milk. It then caused a crusty rash to erupt all over his body, much like that of a leper. He then died. In short, two effects of the poison, in addition to a swift death, is that it causes the blood to coagulate and a terrible rash to form.
The ghost mentions that the poison used is hebenon. It is unknown for sure exactly from what this poison is actually derived. There has been much speculation over the years. It may be ebony, yew, hemlock, or henbane. However, the toxins derived from these do not cause the effects described in Hamlet. It is possible that Shakespeare's poison is a substance of fiction, and its effects are described as such in order to elicit a strong reaction in both Hamlet and the play's audience.