DwD Studios

If you have at least one level of thief you can choose to use your thief score rather than your warrior score when using any weapon that does not have a strength requirement such as a dagger, shortsword or sling. In addition, when wielding a melee weapon, gain +1D damage per thief level when your target is at a disadvantage.

For instance, a level 2 thief is armed with a dagger (1D damage) and your target is not armed, outnumbered 2:1, has his back turned, etc. When you hit cause 3D damage.