I'm making a character that's a stealth/magic hybrid assassin-type character. The basic idea is, he won't use bows. At all. The majority of his attacks will be magic, but he'll still have a sword if he needs to fight up close. Here's the build:

Class: Shadow
Best Race: Dunmer/Dark Elf
Best Birthsign: The Thief
Specialization: Stealth
Attributes: Agility, Willpower
Major Skills:
1. Blade
2. Sneak
3. Destruction
4. Illusion
5. Light Armor
6. Acrobatics
7. Security

Should I change any skills? I'm thinking maybe Security for Mysticism, or Blade for Mysticism. Life Detect is just so useful .

Also, any ideas on spells to use for Destruction? Both Touch and Target, and I don't care if it's a custom spell or one that's bought.

Destruction, chain your spells..

Start with a weakness to magic 100% 4 secs, then another spell weakness to element 100% 4 secs, follow that with another spell damage 15pts element spell, all on touch, that will kill just about anything in game.

Ranged go for Area of effect 15' - 25' for 30 - 50 pts damage to make sure you deal the blow on chargers.

Absorb and drain spells are great use them, fatigue a foe to ko him, drain health 150+ for 1sec is a instant kill, absorb health is a fantastic frontline spell.

Illusion is your best friend, self made command spells at level + 2, upto level 25 for 3 - 4 secs will make any fight a piece of cake keep 4 versions imo, a AoE touch and ranged spell with a single target touch and ranged spell for when needed.

Tag invisibility for 5 - 10 secs on your spells to make sure you don't leave yourself out n the open.

As you're not a dedicated caster, make sure you plan out combat first as you'll end up with not much magicka without proper planning.
Take potions, sneak alot, and use cheap self made commands to do the brunt of your work, then when one on one with an injured npc engage with your other tactics.

For attributes you might want to think about changing willpower for intelligence. Dont get me wrong, willpower is useful, I just like int. more because it increases you dmg and amount of mana you have.

If you substitute one of those for mysticism, pick security. Your security level doesnt make any difference of your ability to pick locks, and I find lockpicking very easy. You might want to think about taking sneak out of majors, because sneak levels FAST, and if you level it too quickly, your character will level up alot, thus making the enemys HP/dmg greater then yours.

For destruction, touch spells are way cheaper to make mana/money wise, but some target skills might help. And yes, making customs spells is a MUST for any type of character IMO.

and yeah, illusion will be your best friend (paralyze, comand, frenzy, etc. are GREAT to have)

Honestly I would replace security with restoration. I always do this simply because of fortify security 100 points for 1 sec on self (some may consider it cheap but lockpicking really annoys me). Restoration also gives you restore health and fatigue which is always handy. If you are having trouble sneaking you can do a fortify sneak, if your blade isn't hitting as hard as you like you can fortify blade etc. If you cant tell by now, restoration is my favorite skill

Just one thing: I read that when casting spells - or making spell with more than 1 effect- yo should place the weakness to element / magic AFTER the actual damage effect.

If magic is to be your main attack, then take the Mage sign to get yourself more magicka. You really don't need the Thief sign boosts.

Most useful early Destruction spell: Drain Health 100 for 1 second. Almost TOO good! Almost. Pick up Turn Undead, too - there's a novice level version, and it's a cheap cast. Minor Life Detect is available at novice level so no need to make Mysticism a major. You're better off using an item anyway, preferably made from a sigil stone. Or you can go get FinGleam.

Wearing armor interferes pretty seriously with Command and other level-dependent Illusion skills. You may want to re-think this.

Mara

You can add weakness to magic 100 for 2 sec after drain health 100 for 1 sec, second cast kills everything with less than 200 health, make it area effect on higher levels and it’s nice to kill lower level enemies.

One spell I love is a combination spell, shock damage x+ drain health 100 for 1 sec + weakness to shock 100 for 2-3 sec + weakness to magic 100 for 2-3 sec

This spell is pretty interesting, if x=25 first cast kills everything with less than 125 health and does 25 in lasting damage, second cast does 50 in lasting damage and kills everything with less than 25+50+2*100 in health. Next cast and the following doubles the shock damage each time as long as you hit before the weakness times out and the enemy dies once his health falls below 200.
Yes casting weakness first and follow up with a damage spell cost less mana but is harder to use in the heat of combat.

Sneak is pretty useful for mages in the combination of a detect life item or spell, then you see the enemy go into sneak mode until you find a good place to cast on him, yes magic show your position far more than arrows but only the target and enemies close to him attacks you.