The Sword of Kas is a very powerful artifact from the world of Greyhawk. It is detailed in the Dungeons and Dragons Third Edition Dungeon Master's Guide, though that is not the first appearance of this artifact.
Kas was Vecna's most trusted lieutenant, until he turned upon him and slew him with this sword. The very fact that he managed to slay this near-deity (a hero-deity in Greyhawk terms, despite being a villain) with the sword shows its power. It's not that surprising, since the sword's creator was Vecna himself. In the old first edition and second edition rules, it had the singular honor of being the only +6 sword ever detailed. (I may be wrong, but I've heard that from others as well). It was an chaotic evil intelligent artifact, able to speak, and had the power of defending as well. In the third edition, it is still an intelligent weapon. It gives a large enhancement bonus to strength, is still a +6 weapon (and still the only published weapon with that large of an enhancement bonus), and is also "unholy," "keen," and "vorpal." In addition, it has the power to cast several powerful spells. (See the DMG for a full laundry list of powers.)
After the epic battle between Vecna and Kas, the sword was left behind. Both participants in the battle suffered "fatal wounds," but since both were undead (Vecna a lich, Kas a vampire), both managed to get better. Later the sword was discovered by the demon Orcus (a near deity) and used by him to gain more power on the Prime Material Plane. Finally, the sword can be discovered by players in the module Vecna Lives and Die Vecna, Die!. (Though I won't give any details to avoid spoiling it.)
In short, if you ever hear a story about a player picking up the Sword of Kas off of a stray kobold at first level, you have just met one of the biggest munchkins in existence.