That's not in the rules, but it's retarded, so you can't do it.

We finished off the Blood Reavers and moved onto the Duergar. Quij discovered that, unlike Alaric, the Duergar are immune to fire. Dave learned never to borrow my dice. Alaric found a life-drinking scimitar, which he named "Thirsty" and promptly used to destroy a duergar caster utterly. Finjaq learned that Piercing Strike is awesome, but just shanking an orc that someone grappled is its own reward. Bakun was late, so he learned NOTHING. And Brynn... well, aside from discovering that hobgoblin arrows suck and will break easily, she learned that the rest of the table has the senses of humor of ten-year-olds. We all learned to hate invisibility, and next game we're sure to have more duergar coming thanks to the one who got away...