Thanks to the revamped leveling system in The Elder Scrolls 5 - Skyrim, you're more or less free from the stigma of creating and sticking with a certain class, and are left free to make decisions on your play style(s) as you progress throughout the game.
Needless to say, this does little to discourage most 'power-players' - those who want the most out of their character build - from deciding early on how they wish to develop their character, and sticking with it.
Throughout Oblivion, I was able to skate by relatively easily playing a stealth/archery focused character. Sneak attacks were outstanding damage, and the stealth aspects of the game added an additional level of complexity to approaching situations.
When playing an assassin you're often left asking yourself a few questions before approaching combat - like wondering where the light sources are, or if an enemy has line of sight to you. Perhaps you're one of those super-patient players that sits still awhile and watches the movement patterns of enemies for a few minutes before deciding the best course of action. I've never been that patient, but thankfully the damage-dealing capabilities of a few of the enemies in skyrim force you to take stock of your situation before moving forward. Failure to do so will often result in reloading your most recent saved games, as failed assassination attempts are usually disastrous against multiple stronger opponents.
Now, while the build I'm about to discuss is (I'm sure) nowhere near the best or most optimized for damage dealing, I feel it poses an excellent representation of the assassin play style that so many of us have come to love over years of gaming.
Starting off is pretty simple, you want to make sure you always have light armor on, this lets you move faster and makes less noise - the lighter your armor, the harder you are to detect. That's important.
You want to take your first perk, when you reach level two, and dump it right into the stealth tree. This will make you 20% harder to detect - enabling you to get many more sneak attacks, and give you much more time sneaking (and raising the skill) before you are detected.
Leveling from here is best done from a melee approach first, and a ranged approach second. Stick with the dagger and a light shield (DO NOT DUAL WIELD). Why no dual wielding? Without the shield to block, interrupt, and mitigate damage, you'll be extremely vulnerable to groups of enemies, and anything (of reasonable strength) that survives your initial back stab.
The fastest way to go about leveling this build is by sneak attacking in early levels. Take your time through smaller dungeons, and stay in stealth (by pressing the control key). Keep an eye on where enemies are facing, if they're walking, and whether or not there's a huge light source you'll have to walk into before getting to them. When you start to get close (15 feet), press the caps-lock key to turn OFF auto-run, and slow yourself down. This is crucial as until you reach higher levels of stealth, you won't be able to run when you're close to enemies unless you want them to have an excellent chance of spotting you.
Approach enemies from the sides or rear if you can, but avoid being in their line of sight if you're in a lighted area, and/or if you're closer than 15 feet. Once you're in range, you can hold the W key down and do a power attack forward with your dagger, closing the distance on your enemy and executing the back stab for crazy damage. This is important to execute correctly, and it takes PRACTICE. A successful back stab will, up until level 50 or so, automatically grant you an additional level of stealth, and thus enable you to level your character quickly. Take everything in the stealth tree, up to the 6x and 15x back stab multipliers as soon as they becomes available.That's your goal - get those perks.
*Note, its not necessary to power attack with a back stab. The damage multiplier comes off the weapon, not the attack type. Take careful note of this, because if the enemy is slow enough to turn around, you can back stab them multiple times. If you can pull this off, you can even kill dragons before they get a single attack on you.
*Also of note, back stabbing dragons is extremely rare, you can spend hours and multiple save attempts trying, often its just easier to bow them down. Random-encounter dragons will just fly away if you stealth too long.
In leveling, you're bound to also get other skill-ups, specifically for one handed weapons from using the dagger. You should spend any extra perks not immediately spent in stealth in the one handed skill tree, in the armsman skill. There are only five ranks in this perk, but all increase your one handed damage by 20% each. That includes daggers, which includes back stab damage.
Once you've got your level 50 sneak skill, you should also get the silent roll perk. This enables you to close the distance on enemies quickly and SILENTLY while tapping the ALT key, and moving forward while running and sneaking. Very useful, but takes some practice to get good with. It's important to realize that this doesn't mean you have to roll in a straight line - you can direct the roll with your mouse, just as if you're sprinting. This lets you roll around corners, away from spells, etc. with some practice.
You'll have extra perks backstabbing your way to 50, and eventually 70 stealth - where you'll get the fantastic perk called silence - which makes running while sneaking viable at all times, as it no longer effects enemy detection.
Once you've got assassin's blade, you'll notice that it's not particularly useful in all situations - no matter how much you want it to be. On your way there, with some of those extra perk points, go ahead and throw a few in archery, overdraw to be specific, and start to level that tree up. This will give you a ranged option to opponents that are out of reach. It will also be your primary damage dealer against dragons and most bosses, as trying to back stab them is nearly impossible without a lot of saving and loading the game.
When leveling, be sure to level Hit Points and Stamina on a 2/1 ratio, respectively. Stamina will allow you to utilize your silent roll more frequently (You won't ever really need more than 200 Stamina, level HP exclusively once you're there), and hold a drawn bow for longer while zoomed in and observing enemies. Both crucial to your survival. Don't bother with Magic (yet). As with all characters, Enchanting is a fine profession to take up if you're bored.
The real shining moments from this class come not only the involved game play of having to precisely time your attacks and offensive strategies, but also in watching your enemies nearly explode when you hit them and receive that 15x damage multiplier. It never fails to bring a smile to your face.
One last piece of advice regards to shouts, the throw voice shout is pivotal to your distraction capabilities. You can not only scout ahead with it by shouting across a room while sneaking, but you can divert attention away from you so that you can bypass enemies, or line up some bow-shots, or perhaps more importantly - you can get someone to investigate something and turn their back to you. Very Sneaky.
And finally, you shouldn't plan on going through extensive melee combat with your dagger. After your back stab attempt, swap out. Either progress up the one handed tree and select a favorite weapon, or go 2-3 perks into the conjuration tree and summon some pain. Conjured weapons are a fine substitute for real ones, although they can not be enchanted - summoning a powerful bound sword only requires two perks up the conjuration tree, for novice conjuration and mystic binding.
I hope you enjoy this build for what it's worth, you're a subtle powerhouse with a lot of burst damage at your disposal. Certainly not for the ultra-impatient, but I'd encourage anyone looking for a change of pace in Skyrim to give this a shot.
