Source: Player's Handbook 4th edition
The path of the shadow assassin is one embarked upon by rogues who seek to become the deadliest warriors there are. Shadow assassins strike from darkness, dealing death and slinking back into the shadows before retaliation can commence. The goal of a shadow assassin is to fell their enemy as quickly as possible, leaving little, if any, chance for their own death.
Shadow assassins, who are similar, but not the same as the more generic assassins, excel best at dealing damage swiftly and most of their training is directed towards in spilling as much blood as quickly as possible. Lowly shadow assassins far less skilled than their more experienced compatriots are nonetheless capable of making strikes with lethal accuracy. Even attacking these shadow assassins can be dangerous, who parry their enemy's blows and then, as the attack regains his or her composure, strike with a riposte strike. And once a shadow assassin has the edge over an enemy, things become even worse, as the shadow assassin takes advantage of their opponent's weakness to finish them off.
The exploits of a shadow assassin are similarly focused on maximizing deadliness. Final blow is one such technique, bringing heavily wounded foes within death's reach so long as the shadow assassin remains close enough to exploit their weakness. Bad idea, friend is, however, a different kind of exploit that instead of increasing a shadow assassin's deadliness, makes them less likely to be hit or affected by an enemy's blows. Both are important to the success of a shadow assassin in the field.