Prestige classes, part of the d20 System that governs Dungeons & Dragons third edition, extended the idea of multi-classing.
The five prestige classes in the first Neverwinter Nights expansion pack, Shadows of Undrentide, are the Arcane Archer, Assassin, Blackguard, Shadowdancer, and Harper Scout. They have various requirements, such as must be non-evil (Harper Scout), must be elf or half-elf (Arcane Archer), and must have 8 ranks in both Hide and Move Silently (Assassin). The new prestige classes in the latest expansion pack, Hordes of the Underdark, include the Champion of Torm, Dwarven Defender, Pale Master, Red Dragon Disciple, Shifter, and Weapon Master.