Black Desert (formerly Black Desert Online) is a sandbox MMO first released in the west in 2016. As a Korean MMO, it's got an in-depth character creator, better-than-average combat, and a reputation for extreme grindiness. While it's free-to-play in Korea and a couple of other countries, on Steam it would normally set you back $10. To celebrate the release of its Eternal Winter expansion, Black Desert is available for free until April 13.
Grabbing it for free nets you The Traveler Edition, which includes the Eternal Winter expansion. Black Desert's biggest expansion to date, it adds the Mountain of Eternal Winter region and a new class called Drakania, who wields a rather long sword and armor forged from the scales of slain dragons. There are new boss fights, endgame locations, and also ice fishing. Plus there's a season server for new players, which will apparently speed up progression with a variety of rewards and buffs to help newbies get to the latest additions.
Back in 2017, Austin Wood wrote that Black Desert isn't a great MMO, but it is a great sandbox RPG, saying that, "Black Desert lacks much of what you might expect from a modern MMO, but what it has in spades is agency. This is a game about building an empire, more akin to Civilization than World of Warcraft. At least, it is to me. To you, it might purely be an action-RPG with combat more fluid and frenetic than any other MMO. To many others, it's a captivating role-playing tool powered by a rich character creator and a thickly detailed world."
A few years' worth of updates later, Black Desert eventually made our list of the best MMOs under the sandbox section, where we noted, "It can be just as rewarding to spend an evening tweaking your farms and leveling up your workers as it is taking down one of Black Desert Online's brutal world bosses. And if that doesn't suit your fancy, the node system is also the foundation for weekly guild wars, where guilds race to conquer various nodes for special bonuses—making BDO a great choice if you're into PVP as well."