Legend has it, that the bearer of the mask will gain great powers, like the "Gaze of Death". It is an amalgam of gold and sourcers and was worn by Medusa herself.
First Lara is not convinces, that the Mask exists at all, that it is more than a legend.
The last time the medusa mask was seen was 1505. The owner, a rich man, was travelling on a Spanish Galleon to America. The ship sank in a violent storm. The exact location was unnknown.
D'Arseine claims, that he knows where the ship sank, but he also says that the mask was taken away from the wreck.
Chase Carver has left a trail at the wreck which Lara follows. She finds him, but he has only found a message in a bottle. According to this message some kind of madness took over the ship. One of the sailors flees with the treasure, the medusa mask.
More on Medusa