Increasingly, though, I've begun to think that magic items should be more common than magicians - for the simple fact that magicians (along with dwarves and elves and other supernatural creatures) make them - and probably make several over the course of a career. But rather than providing unqualified benefits, magic items should be - at the very least - double-edged. Here's one that is - quite literally.
This is a plain-looking longsword with a grip and scabbard bound in a peculiar soft leather (human skin, in fact). When drawn, the blade reveals "veins" of blue and red in the metal. In combat, it does 1d20 damage (1d20 + 1 if wielded in two hands). When it draws blood, the veins on the blade writhe and pulse, and the sword appears to suck noisily at the wound.
Once drawn, the Drinker cannot be sheathed until it causes a wound. It poses no threat to its owner when combat is underway, even if that combat breaks into successive skirmishes. Basically, if the wielder can still see living enemies that he can hope to attack, he's safe. But when there are no more foes to fight and the sword has not caused a wound since being drawn, it will twist in its wielder's hand to deliver 1d20 in damage
Any attempt to avoid this by throwing the sword away or otherwise discarding it will prove futile; the Drinker will slake its thirst before it will allow itself to be abandoned. The wielder can, of course, avoid this by turning on a friend, though friends are likely to prove scarce if this becomes common practice.
When it has caused a wound, the Drinker becomes docile and is easily sheathed. If it is not drawn during a fight, however, it will sigh and groan in its scabbard - sufficiently loudly to attract nearby creatures. The Drinker thirsts only for the blood of the living and cannot be assuaged with even the most freshly slain corpses.