Yes, it's op. Everything is op.

Now that that's over with, here's the build:

Get your illusion trained and pick the perks Kindred Mage, Hypnotic Gaze and Dual Illusion.
Get the Stealth perk Backstab. Get Assassins Blade if you prefer daggers.
Get the Calm spell.

When fighting against enemies cast calm on them. Then stroll around to their back, sneak, sip at your coffee, attack.
If enemy not dead yet, calm again.

Augment that "combat" approach by enchanting your gear to make Illusion spells cheaper.

Greet your enemies with: I COME IN PEACE!

I feel this build is great, although in the end, it loses usefulness. I can stand infront of most enemies with 100 sneak, all perks in the tree + stealth enhancing gear and they don't see me. If they do, I have a stack of 40 invis potions to sort the situation out.

You have discovered the gamplay of the Vampiric Assassin Clan. The secrets are out!

EDIT : Actually all you need is the Invisibility spell and a high sneak skill. Add in the Assassin's Blade perk and you will never die. No potions...no armor...no problem.

Any decently built character should dominate the game after level 40 on master difficulty.
This is just another way to comfortably get there.