A small slab of polished granite about 7 inches long by 3 inches wide by half an inch thick, with hieroglyphs engraved on one face. The hieroglyphs are recognisably part of an ancient language, now unreadable to all but a very few (highly specialised) sages. There are no other identifying marks.
There are no moving parts or mechanisms of any king. However the slab does radiate a faint magical aura, which is visible to those who can detect such things.
When any of the hieroglyphs is touched the slab emits an audible "beep", of the type that would be called "electronic" in a more technical universe, but nothing else appears to happen. There is no variation in tone or note between the hieroglyphs, all of the hieroglyphs producing exactly the same sound.
The slab is actually the control unit to an ancient magical device or mechanism of some kind (think TV remote control). This can be almost anything the GM can envisage (a slave golem, an air conditioning system, an entertainment device, whatever). Unfortunately, the device is long since lost so the controller now does nothing at all.
Oh, and the beep sound was included by the original enchanters as a kind of "feedback" to confirm to the user that a hieroglyph had been touched.
Just drop this into a treasure hoard somewhere then sit back and watch your players get really confused.