Centuries ago, Merlin conferred upon a collection of gems "the power to make their owner invincible." These gemstones, later known as the Merlin Stones, would be sought after and used as sources of power by various beings over the following centuries.
In the 17th century, the stones were owned for a time by pirate captain Red Lucy Keough, an ancestor of the Scarlet Witch. Red Lucy decided she didn't want them and left them in the hands of her crew.
Learning that Blackbeard was once in possession of the Merlin Stones, Doctor Doom sent the Fantastic Four back in time to retrieve them. The Fantastic Four found Blackbeard's treasure with Red Lucy's old crew (and discovered that the Thing was in fact the one true Blackbeard himself), but let the treasure sink to the bottom of the sea in order to keep it out of Doom's hands. They returned to the present with an empty chest and nothing more.