Best part of this build, is getting Bat Swarm III at level 13. That is some serious tech.
In all seriousness, I have recently stopped using a Hybrid build for Erebus (Minions/Assassin Hybrid) that was pretty much this build, though twiddled around a bit. The main difference is that Hybrid Erebus gets a little mass charm, and a little coven, while assassin gets only mass charm, and Minions get no Mass Charm. No matter how you look at it, the amount of Charm is the real difference.
Oh, and I think that Bat Swarm II is better than Bite IV for both Hybrid and Assassin. I do agree with Mist form however being something you need to know about as an Erebus player. You do not necessarily need it anygame, but you need to be fluid enough with your build to know WHEN you need to grab it, and then actually grab it. A short mist can be amazingly effective at helping you survive.
Lastly, I think people need to remember that mana is very important to Erebus. Unlike some DG's who need to keep their HP high via HP items, Erebus can almost always escape if he has enough mana for batswarm, and mid combat can bite someone to death as well. Keep that MP up!