Skrim Assassin Build?

The best assasin i would say is wood elf. They get a good archery bonus to start with. I dont know what you mean by school of magicka? is that like conjuration and stuff? if so i would do conjuration, its godly. the best enchantment is banish

1. Use your brain, make sure you spec into sneak, pickpocketing, and one handed. Lockpicking perks are VERY unnecessary, after two assassin characters I have found that, even with master-locks, it is easier to buy more lockpicks to get it done than waste the perks needed to keep up with your misproportioned leveling, more on that later

2. Only spec into magic if you want to be a mage, it is difficult to manage getting all the perks you need with such a division of interests

3. If you truly want to be an assassin, go into the dark brotherhood, both the gloves and boots you get there are EXTREMELY helpful as an assassin, with being specced into sneak with the "15x Dagger backstab damage" perk and the gloves, you will do 30x damage from a dagger sneak attack.

Finally, if you are going heavily thief, leveling pickpocket basically, you will find yourself leveling inordinately fast, if you don't take the time to level and spec heavily into one-handed then you find yourself at a major disadvantage as you fight monsters that have scaled to your ballooned level, and it becomes strangely difficult.

My 2 cents

for an assassin? i reccommend either the Argonian or Khajiit, as both are more associated with thieves and murderers.

next you want to study in Destruction, Restoration, Illusion, and Conjuration as this was my magic studies as a assassin and thief.

next you want to pour perks into One handed, sneak, archery, lockpicking, speechcraft, and perhaps pickpocketing

all are necessary skills for the murdering thief

and enchantments? mainly ones that make it harder for you to be detected while sneaking, enchantments that increase one handed power or archery power

most of this is readily available just by joining the dark brotherhood or the thieves guild