Essentially, you can make a Killing Blow X times per day. They occur under the same circumstances as a sneak attack, and are basically coup de graces - automatic critical (if they hit) and the target has to make a Fort save vs. 10 + damage inflicted or die. Oh, it provokes AoOs from everyone but the target, too (until 14th level.)
I haven't playtested it - gut instinct says its on the border of too powerful (first level assassin, weilding a short sword, average damage means a Fort DC of 17!) On the other hand, the AoO and the limited uses per day are pretty big balancers.
My chief worry would be synergy benefits between the Killing Blow and other class abilities you gave the PrC, or other class abilities that they are likely to have when entering the PrC.
For instance, a high sneak attack bonus makes the Killing Blow even more effective, so if it's going to be rogues going into the PrC I'd be very careful what else you give them - if they have the ability to set up their own sneak attack/killing blow situations easily, that will multiply their power a great deal.
J
