Soul of the Ultimate Nation (often abbreviated as SUN) is a fantasy-based massive multiplayer online role-playing game (MMORPG) produced by Webzen, a Korean-based company. It is operated in Korea by WebZen and in mainland China by The9. The game was launched in Taiwan on April 19, and in China on May 24, 2007 as a paid online service.

In December 2005, WebZen sold an export contract for the game to The9 for $13 million USD in total, which at the time was the largest single export transaction in Korea’s online game history. In 2006, SUN was selected as the most anticipated online game at the 2nd Annual ‘Best Online Game Selection’ in China. In a survey by Gametrics, the largest Internet cafe survey website in Korea, SUN was ranked among the top 10 games in Korean Internet cafes.. Also, SUN received the Award at the Digital Contents Awards by Ministry of Information and Communication and SK Telecom in 2006.

Like some other MMORPGs, SUN utilizes the "hack and slash" way. Players are part of a rebel group known as "The Guidance". Items can be strengthened and upgraded throughout the game play of SUN. For large-scale combat such as siege warfare, Soul of the Ultimate Nation utilizes a combat map with partitions that divides players into multiple fields each containing 20-40 characters; the outcome each battle having direct affects on the combat occurring on other fields. SUN also allows players to strategically set the stage for battles by choosing different types of maps, entry numbers, degrees of difficulty and types of monsters for a customized game play experience. SUN has three types of specified maps using a zone map system. Players start the game in a city and have to create zones to play the game. In terms of maps, the mission maps are the primary places where players can solve missions. The hunting maps are for tracking down monsters to level up characters, gather items and acquire in-game money. The quest maps are used to fulfill quests and to help users better understand the overall scenario of the game.

The game has already undergone four stages of beta from September 2005 to May 2006 in South Korea, and received high marks from gamers. SUN Taiwan moved from closed beta to open beta on 20 December, 2006. The current open beta is allowing the game developers to improve the field maps, add a competitive hunting system and a mission ranking system. SUN Korea adopted F2P mode and item mall model on November 14, 2006. NHN USA Corp., the publisher of SUN in North America which made a license agreement with Webzen on May 21st, 2009, has not decided whether or not they will adopt the same model. SUN is expected to be available for distribution and operation in North America in 2009.

Under the Battle Zone System, players can create "rooms" for battles, be it against monsters or other players. Players will have to talk with a specific non-playing character, (also called NPC`s) in each town to create such rooms. They can select different types of maps, set the entry numbers, degree of difficulty and types of monsters within the map. For a players vs. monsters map, players will form a party and usually have to proceed through the map to complete certain map objectives, such as killing the boss monster. At the end of the stage, experience and loot are shared among the participants. More quests and missions will be added to SUN to continue the ongoing story. Additionally, when a mission is near completion, tips will appear for readers to unlock hidden missions.