Dota 2 is a free-to-play multiplayer online battle arena (MOBA) video game developed and published by Valve Corporation. The game is the stand-alone sequel to Defense of the Ancients (DotA), which was a community-created mod for Blizzard Entertainment's Warcraft III: Reign of Chaos and its expansion pack, The Frozen Throne. Dota 2 is played in matches between two teams of five players, with each team occupying and defending their own separate base on the map. Each of the ten players independently controls a powerful character, known as a "hero", who all have unique abilities and differing styles of play. During a match, players collect experience points and items for their heroes to successfully battle the opposing team's heroes in player versus player combat. A team wins by being the first to destroy a large structure located in the opposing team's base, called "The Ancient". Development of Dota 2 began in 2009 when IceFrog, the pseudonymous lead designer of the original Defense of the Ancients mod, was hired by Valve to create a modernized sequel. Dota 2 was officially released for Microsoft Windows, OS X, and Linux-based personal computers via the digital distribution platform Steam in July 2013, following a Windows-only public beta phase that began two years prior. The game initially used the original Source game engine until it was ported to Source 2 in 2015, making it the first game to use it. The game has also been updated with various other features since release, such as support for virtual reality (VR) and a data-providing and achievement-based paid subscription system. Despite some criticism going towards its steep learning curve and complexity, the game was praised for its rewarding gameplay, production quality, and faithfulness to its predecessor, with multiple gaming publications later considering it to be one of the greatest video games of all time. Since its release, Dota 2 has been one of the most played games on Steam, with over a million concurrent players at its peak. The popularity of the game has led to official merchandise for it being produced, including apparel, accessories, and toys, as well as promotional tie-ins to other games and media. The game also allows for the community to create custom game modes, maps, and hero cosmetics, which are uploaded to the Steam Workshop and curated by Valve. A digital collectible card game spin-off by Valve, titled Artifact, will be released in 2018. Dota 2 has a widespread and active competitive scene, with teams from across the world playing professionally in various leagues and tournaments. Premium Dota 2 tournaments often have prize pools totaling millions of U.S. dollars, the highest of any eSport. The largest of them is known as The International, which is produced annually by Valve and most commonly held at the KeyArena in Seattle. Valve also manages an event format known as the Dota Pro Circuit, which are a series of tournaments held annually that award qualification points based on results for getting directly invited to The International. For most tournaments, media coverage is done by a selection of on-site staff who provide commentary and analysis for the ongoing matches, similar to traditional sporting events. Broadcasts of professional Dota 2 matches are streamed live over the internet, and sometimes simulcast on television networks, with peak viewership numbers in the millions.