Examine the altar

You walk over to the altar and examine the golden box. It looks like it's made from solid gold, a hypothesis which is supported by your inability to lift it. Despite this, you see that there's a keyhole on the side facing you, though by the lack of seams or hinges, you have no idea how the box will open. You take out the key you found and insert it into the keyhole. Slowly, you turn the key.

At first, nothing happens. But as you complete a full circle, you hear grinding coming from the box. You jump away, just in case the box explodes. You see hairline fractures appear across the box and it disintegrates, leaving behind a pile of gold chips.

You approach the altar again and look at the contents of the box. You see a small cylindrical object, some sticks of incense in a dish and a flint to light the bouquet of incense. You want to appease the god and get his help, but you're not sure how.