Darius decreed that anyone who altered his edict would be put to death on what?

Answer: A beam from his own house

Darius ordered that a beam be pulled from such a man's house and he be put to death on it, and his house made a rubbish heap.

“I also decree that if any man interferes with this directive, a beam is to be torn from his house and raised up, and he is to be impaled on it. And his own house shall be made a pile of rubble for this offense.”— Ezra 6:11 (BSB)