To a large extent, it reads like the caster is using a zombie as his familiar. Sufficiently so that I'd be more inclined to build it off Find Familiar as either a specialised or higher-level version than making it a separate spell.
That is actually a really good point! I will need to reread the Find Familiar spell and see if this can be adjusted or improved based around that spell.
+1 for depravity. It reminds me of the wargs from A Song of Ice and Fire, so you might look there for inspiration. Also, it would be pretty dang cool if the zombie's consciousness took over the caster when the caster took over the zombie...
Zed the Necromancer: Brains...
Eddie the Fighter: Guys! He's doing it again!
Karl the Cleric: Ow! He's on top of me! Should I turn him or what?