Disc/Shadow

This build starts with shadow, and once it get Shadowform at 40, switches to disc until it tops out with Rapture at 80, the build boosts damage, and survivability all the way up this path. Its super fun to play, with only one caveat, this a a water suck build, mana control is a primary concern, and routine hydration is mandated.
At higher levels it uses the spec'd out PW:Shield as a virtual heal without breaking shadowform. Face melting (Mind Flay spamming) behind a powerful shield is the order of the day. Mind Blast and Devouring Plague are spec'd out too
Gear towards spell damage, and crit chance. Stat Stam over Int.
A big HP pool will make up for not sinking 20 points to get Dispersion.
Do you want Focussed Mind instead of Rapture? It allows for smoother farming, but lowers overall survivability, the 4.5% mana bump occurs when you need it most, so it is extra handy.