https://www.polygon.com/2018/5/29/17402932/sea-of-thieves-content-teams-expansions-weekly-events

The Hungering Deep is the first new major content addition to Sea of Thieves, and it’s a direct response to many of the criticisms and complaints the developers fielded from the base game. The new campaign, set of cosmetics and AI threat is more than just an event; the new mechanics represent Rare’s decision to hand the tools to the players and allow them to remain the main characters in a chaotic world of player choice.

Sea of Thieves’ first expansion pack is meant to build upon the successful elements of the game so far. The campaign, given to players by the hideously unlucky pirate “Merry” Merrick, will have players solving puzzles and working together to solve what, exactly, lies in the deep. The campaign, and its accompanying cosmetics, will be limited time, as its meant to reward players for being there at launch and give them a tour of the new content with style.