Yay a shadow priest!. Firstly remove the chance for void tendrils. make it instead summon when you activate voidform but maybe summon 2-3 tendrils ( potential zoning effect). I would personally reduce the initial damage of shadow word; pain but increase the dot and make mind flay a channel similar to Gul'dans life drain. the slow can then potentially be a level 4/7 talent. The cooldowns are also a bit lenghty with you only being able to cast a single mind blast and void bolt during void...

Thanks =D