I've had trouble with the thief's backstab ability (in combat) in the past, especially when playing without tokens or facing on the map. It has the potential to be either overused or underused, depending on the interpretation of the DM.
What I've been using recently, and it has never been written down prior, as DM fiat required me to figure out a solution on the spot, is as follows:
- if the target is engaged in combat with another
- the PC attempting the backstab has not engaged the target prior
the PC may spend one round getting into position. on the next round, the PC rolls a Hide in Shadows attempt, and if successful, he gets the bonus of a backstab. If he fails, he gets a +2 bonus to hit, but no backstab.
It works thus far, and may come up once per session (if that), usually against a BBEG.
How do you handle backstabs? Do you even allow an attempt in melee?