https://motherboard.vice.com/en_us/article/597mg5/one-of-the-biggest-games-in-the-world-is-a-mod-of-a-mod

Dota 2, like most video games, has you play the hero. Dota Auto Chess, a wildly popular mod of Dota 2, reimagines you as the head coach or general manager for a team of heroes. It features the same cast of characters—a dragon knight, a druid—but now you're deciding whether you have money to put them in your lineup, or whether or not they even belong in your lineup. Essentially, it’s the drafting phase of Dota 2, where you pick out what heroes you and your team will use before the game begins, spun out into its own complex, interesting game.

Just months after its release in early January, Dota Auto Chess has captivated both Dota 2 and non- Dota 2 fans. 870,233 people were playing Dota 2 concurrently at the time of writing, more than 310,000 of which are playing Dota Auto Chess. It’s only fitting that Dota Auto Chess was built as a mod to of Dota 2, a game that was originally born as a mod of another game, Warcraft III: Reign of Chaos. Like many popular mods, the success of Dota Auto Chess has been based purely on word-of-mouth; players saw their friends and peers trying out the mod and it spread from there.