If you want to play The Legend of Monkey Island DLC from July 20, you're going to need the base game Sea of Thieves. And if you don't have it yet, to celebrate its 5th birthday, Steam have it on sale at half-price, as well as the other related soundtracks, wallpapers etc, for the next day.
Go buy it the instant you have Dialed up your Destiny. Or now. Whatever.