Ugh...a hybrid. But I have to say, after playtesting it a few times today, it tops out much more powerful in the endgame than a straight assassin Erebus. Here's how it looks:
1) Bite
2) Coven I (Mist I will be taken later, but early on the extra 4 Nightwalkers will be your best creep killing tool, and are also fairly useful vs. other DGs)
3) Mass Charm I (only for the anti-teleport stun to get kills)
4) Bite II
5) Bat Swarm I
6) Mist I
7) Bite III
8) Poisoned Blood I
9) save
10) Bite IV / Bat Swarm II
so far, nothing out of the ordinary...
11) Improved Conversion Aura I
12) Coven II
13) Coven III
14) ICA II
15) ICA III
16) Army of Night
17) Mist II
18) Mist III
19) Mist IV
20) Bloody Haze
What's going on here? Well, up to level 10, you're nothing terribly different from a standard assassin spec Erebus. You've got your six nightwalkers and I'd recommend priests and siege archers, but bite is maxed, Bat Swarm II is reached ASAP and the standard anti-DG abilities are all in there at their first (and best) level. You try to end the game at the midgame by the standard method.
If this doesn't work out, however, this build allows the option of switching tracks to a heavy minion build in the mid-late game, with a fully assembled army by 16th level. If it gets that far, you end the build by moving up to Bloody Haze and going heavy mist in support of your minions/creeps in the endgame, which I'm starting to see more and more is generally Erebus' best tactic in a stalemated endgame. At some point along the way, you exchange typical 1 v. 1 gear for minion buffing gear and enough mana regen to get funky with mist for long periods of time.
I haven't playtested this extensively, so I can't swear that it's a better lategame progression than anything else, but "in theory" it should be, without sacrificing early game 1 v. 1 ability. Can a midgame flip-flop for Erebus be the best way to save him from irrelevance if you can't put things away quickly? Magic 8 Ball says "quite possibly".