In short, if you as a game designer want an invulnerable NPC, make sure that it actually is. The Lord British Postulate was coined in this WoW Insider post, and is the cornerstone of the creativity and ingenuity of the more cunning player. In short, if a NPC cannot be killed through normal means, then there must be some abnormal way to do the job. Quite often, if a character's weapons fail to do the duty, then the task falls upon other NPCs, world objects, or the environment itself to assassinate the relevant target, with some prodding from the player characters. The Lord British Postulate is not restricted to MMORPGs, but can be applied elsewhere: CRPGs, FPSes, even Tabletop RPGs.