The event you might be referring to is the death of King Henry VIII of England's son, King Edward VI, or potentially another figure.
However, the detail about a "red-hot poker inserted into the anus" is a myth tied to a more obscure event, possibly based on fictional or exaggerated retellings rather than historical facts.
Historically, the death of kings or other royal figures often became wrapped in dramatic and gruesome narratives to fuel public fascination, but the example of a red-hot poker being inserted as a means of execution seems to be more legend than actual historical record.
If you're referring to a specific claim or instance, could you clarify which king or historical period you're interested in? That would help provide more accurate and grounded information on the matter.