At some point, when the PCs find treasure, they notice some of the gold pieces are in a currency they don’t recognize. There is a skull stamped upon the coins. They radiate no magic, and there is nothing out of the ordinary about them. No one else in the domain they are in recognizes the symbol.
A week after they take the treasure, any PC who has one of the coins starts having dreams of their past at night, reliving memories. In the background of these dreams skulks a tall woman with a large top hat. Where her face should be is only a smooth expanse of skin. She’ll open a container (drawers, boxes, sacks) and pull a bone from it and then disappear. She’ll do this every night in a different dream.
She cannot be approached and does not react to the PCs.