what would be a good assassian, but with magic? i got the assassian part down good, but i want to be a mage at the same time? any help would be appreciated thank you.

Illusion has the most synergy with stealth characters. Muffle and later, Invisibility can take already proficient sneak skills and make them phenomenal. Frenzy and Calm can be very useful in handling groups.

For my Nightblade, I also like Alteration for shield spells and Paralyze and Conjuration for bound weapons (though that's more of a flavor thing for my character).

Illusion is tailor made for assassin mages.

I actually play an assassin archer necromancer using a conjured bow.

Conjured bow with the mystic binding perk is good even at level 50+ on master. You can find the conjure bow spell even at level 1 for free if you go to fort amol prison, it is hidden in the first room inside a bucket with a lantern in it, pick up the lantern and the spell book with be underneath. There is a map in the room behind the throne room in dragon reach, if you activate the map flag on fort amol, it will add a map marker for the fort on your map. You will have to be able to take down the mages that guard fort amol however.

What I usually do in a fight is sneak up to a group of enemies and cast frenzy and if they come after me I cast invisibility. When they start fighting each other I use the marked for death shout to get their armor down and pick them off with the bow. I raise the dead when they fall.

This is the bare minimum for the build I use: http://www.ign.com/builds/the-elder-scrolls-5-skyrim?d=0000000000000000001111101111111100010111000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000051110131100000000000000010000011

This is my current build: http://www.ign.com/builds/the-elder-scrolls-5-skyrim?d=0000000000000000001111101111111100010111000000000000000000001110000000001010000003500011100110001000000000000000000000000000000000000000051110131151000100010000010000011

Be advised that it requires 593 magicka to cast dead thrall without conjuration cost reduction gear since I did not spend any perk points for conjuration spell cost reduction. It's not needed really since most necromancy spells are relatively cheap magicka-wise. But you should really aim to get at least 50% conjuration cost reduction. As for point distribution, I go 50/50 magicka/health, I don't use stamina much since I rarely zoom in or take advantage of the time slow from steady hand.