Reputation
Not really a skill, per se, an Assassin's deadly reputation can sometimes make attackers rethink their choice of target. Reputation cannot be practiced. It must be earned...
Assassins earn 1% through a successful Insightful Strike.
Assassins lose 1% when they flee, even when caused by a spell (Dread).
Max that can be earned at Hero for Assassins is 75%.
You can check your reputation by typing 'slearn reputation' (though it won't show until you achieve at least 2%).
Black Circle Initiates also get a certain reputation (20%) due to the nature of the Circle but this is not earned and can not be lost.
What the skill does is when you get aggied by a mob there is a chance (dependent on the % of reputation you have) that the mob will turn and attack one of your group members instead of you, so if you had 100% reputation in theory you would never get aggied.