That Cupe De Grace thing...

Can it be used on other players, or is it a PvM thing only?

And, how do you force it to happen. When an opponent is asleep or prone you sometimes get the opportunity, but not always. What warrents it? Not sure if it works on PCs.

The opponent must be asleep, IIRC. It only affects creatures of 4 (perhaps 5) or less Hit Dice. Perhaps that's why you've noticed it working only sporadically.
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It's rogue, dammit, ROGUE!!!