Developed by Blizzard North, Diablo 2 is the sequel to the highly successful Diablo. Released for Microsoft Windows and Mac OS in 2000, by 2001 Diablo had become one of the most popular online games to hit the market. Diablo fans could play the game as a single player or with multiple players utilizing LAN, but the most popular mode of playing was multi-player using Battle.net. Selling over seven million copies, it became the number one best selling RPG for the PC and the 9th best selling computer game. The Diablo gamer assumes the role of the protagonist, a hero who fights monsters in a mythical land full of dungeons and other fictional hazards. The player can choose from five different hero types namely, Necromancer, Amazon, Barbarian, Sorceress and Paladin; all these heroes have distinct characters, traits, strengths, skills, and weaknesses. Apart from different hero characters, the player also gets to choose their preferred level of difficulty from levels such as Normal, Nightmare and Hell. To get through any level, the player must battle a series of amateur monsters and just before being passed to a higher level the player must battle the biggest monster on that level. The player may also hire a computer-controlled mercenary, one of many available on the game, to accompany the player through his or her journey in the game and assist fending off nearby enemies. Whilst on the journey, the player might come across a rare or valuable part of a set; on complete assembly, this set proves to be a strong asset. These valuables are customizable by utilizing gems and sockets and can also be transmuted into different items using the 'Horadric Cube'. An expansion of Diablo 2 was launched in 2001 called Diablo 2: Lord of Destruction. Where Diablo 2 is played through four stages, Diablo 2: Lord of Destruction has five. The Guinness Book of World Records awarded Diablo 2 for being the fastest selling computer game ever as it sold more than a million copies in the first two weeks alone.