The assassin is a deadly melee class with a pet component only available to evil characters. They're a stealthy scout class, deriving their power from their agility score. They use light weapons that they poison themselves to dart in and out of the shadows, dealing death from the flank. They can put in a lot of damage in a very short period of time, although they lack somewhat in staying power. One of their later abilities puts them into stealth with every successful ability that they land, making them very difficult for monsters to get a handle on.
Some of the assassin's abilities like Caltrops debuff their resistance to poison and slow them down, making them easier prey for a clever killer. Strength, agility and stamina are all great stats for an assassin to work on, along with intelligence to improve the proc rate of their poisons. One thing to keep in mind as an assassin is that your ability to do damage outpaces your ability to avoid gaining threat. Try to make sure that the tank has as much hate as possible before you unleash your full attack. Your ability to do damage will be almost entirely dependent on the tank's ability to generate threat. Group with those who can help the tank be more effective while buffing your damage in turn if you really want to be able to let loose. The ability Friendly Shadows, which transfers some hate onto another target, can be particularly useful. Use it often, before you think you're going to have a problem, to make sure that your tank will be able to maintain the attention of their target.
In raiding, an assassin's primary ability is simply to do lots of damage to a single target. Their positioning is very important, as many assassin abilities require you to either be behind or on the side of what you're attacking. Use abilities like Numbing Strike, which roots the target in place, to control enemies more effectively. Assassins also have effective damage over time abilities that can be highly effective, increasing their damage over the long haul on a single target. They also have some area of effect abilities, but exercise caution when using them, as often times drawing aggro as an assassin is a death sentence. Assassins can also apply poison to the weapon of an ally, an ability that should not be neglected when you're trying to maximize the amount of damage that your group does.