Dagger of Baud. It is a very nice dagger and can kill any one creature with a successful hit. But after that, the poison is gone and it becomes a regular dagger. Or if that's not enough, perhaps it can recharge (once a day) or have a limited number of charges (like a wand). It would be handy for getting rid of the pesky Orc Drummers (before you're strong enough to kill them with three hits). You'd need to make some boss monsters immune to it or make it so that they only take 50% damage for balance.
