Hello all... I'm currently on my first play through of Skyrim, (I know better late than never) I have an Illusion based assassin, Level 40, Sneak 93, Illusion 95, and One Handed at 77 being my primaries. Archery (62), Enchanting (4/5 enchanter) and Smithing (63 arcane smith) my secondaries. With Illusion at 75+ with a peerless illusion ring and nightingale hood I'm pretty much able to spam invisibility and backstab just about everything in sight. I was thinking about making destruction my focus from here on out, enchanting some equipment for reduction cost, magika regen, etc.

So my question to any and all of you vets, is this viable, being halfway through the leveling or would I be spreading myself too thin? Thanks!

Not at all, it's totally viable as long as you buy some stronger spells right away (cause at lv 40 it isn't viable to start with flames). You could even incorporate it with illusion and sneaking and use runes as your main spell.

Illusion and sneak will still be my mainstays, I just wont be slitting throats as a primary means of "disposal", something more along the lines of what Festus Rex told me to do. I'm not sure I'll even need to go past adept for a perk?

My suggestion would be to get Illusion and sneak to 100, get a few master illusion spells (eg.mayhem and harmony) and then focus on destruction. You could start using rune spells, which are a nice compliment to illusion/sneak. I've been doing this with my Illusion mage (Cate) who has 100 illusion & sneak and is now working on her destruction.

My reasoning being that once you get to the maximum of a skill, you can keep it whether you are overall level 40 or level 80 - you'll always be as powerful in that skills as is possible and have that at your disposal to use on the tougher bosses that appear at higher levels. Once you get to skill level 100, you can start on another skills set because the ones already at 100 can't get any stronger.

An example might be to work your way through a dungeon using destruction on the lesser dragur types, get to a room full of death lords and the boss at the end and revert to using illusion/sneak/backstab.

You can keep your illusion enhancing gear in your inventory to put on when needed, however you might get high enough magika not to need it.

If you have many skills at less than 100, you run the risk of your character having nothing strong enough for the harder bosses.

Another suggestion for when you get started on destruction would be to work on your enchanting so you can get destruction cost reduction gear. I found that cost reduction gear is needed much more with destruction than illusion.

If you're planning to sneak with Destruction be sure to get the 'Quiet Casting' Illusion perk. You'll be able to silently cast fireballs (or whatever), unnoticed.

Great stuff, this is just what I had in mind. I have quiet casting, but will grab master illusion for mayhem. I haven't worried about shadow warrior, and will grab novice, apprentice, adept, dual, and impact to start.