Arthas is the son of the King Terenas of Lordaeron in the game Warcraft 3. Technically speaking, he is a Paladin hero who starts at level 1 and progresses to level 10 by the end of the Human campaign. Arthas, after discovering that a plague is turning humans into zombies, crusades throughout Lordaeron attempting to stop the undead cult behind the plague. After slaying a necromancer named Kel'Thuzad, he discovers that the true threat is a demonic one, a Dreadlord by the name of Mal'Ganis.
Arthas chases Mal'Ganis to the icy land of Northrend and finds the sword Frostmourne. He uses Frostmourne to kill Mal'Ganis, but discovers too late that the sword is cursed. He succumbs to darkness and in the next (Undead) campaign he is a Death Knight. Ironically, it is Arthas' efforts that allow the demon king Archimonde to invade Lordaeron. At this point in the game Arthas' role lessens, until the Night Elf campaign when he convinces the Demon Hunter Illidan to permanently morph into a demon to slay his nemesis Tichondrius.
It is suspected that because of the open-ended nature of Arthas' story that he will be a character in either World of Warcraft or a possible expansion pack to Warcraft 3.