I'd like to post a few random, disjointed thoughts on my favorite DG, and maybe address a couple of points on which I seem to disagree with many of the design/build suggestions I have seen.

1) Bat Swarm III is mostly useless:  Many builds I have seen suggest upgrading from II to III as soon as possible.  Why?  Level III only increases damage by 150, which at that point in the game is negligible to DGs and you're way past farming creeps, for the cost of 300 mana.  There is no range increase.  Bat Swarm is one of Erebus' bread and butter skills, but many people seem to assume that maxing it immediately is a priority without actually working out the cost/benefit of the upgrade.  Any of the other level 15 skills should take precedence over Bat Swarm III.

2) Mist is an excellent skill: Two reasons for this.  First, it is useful early game for farming creeps if you can catch two waves in the same place.  If you're dealing with one wave only, autoattack is a better idea because you're not wasting mana, but if you catch a mess of creeps together in the early game, a nice simple Mist I can be highly effective.  Smart opponents will seek to deny Erebus creeps in the first 3-4 levels before he starts getting really tough, and Mist I can be a help in early farming efforts which might otherwise be difficult.  Also, a single level of Mist is useful throughout the game as a way to get rid of debuffs/DoT effects like Maim, Spit, etc. and to avoid slow attacks, most specifically hammer slams.  I find Mist I a much more useful buy at level 2/3 than coven or improved conversion aura.  Erebus' great strength is manueverability, and a single level of Mist adds considerably to this.  Mist scales poorly through the mid-late levels and so few people invest in it, but the final skill, Bloody Haze, in quite useful late-game for the 10% evasion, alone.  Autoattack damage from guys like Reg and UB can get stupid late game, and 10% evasion is certainly useful in this situation.

With the exception of the minion-heads (who I don't really understand) most Erebus players seem to invest their "extra" skill levels in the minion skills, but then not really use their minions much, anyway.  Basically, they are throwing away a few skill levels because they can think of no better way to spend them.  Fuck, if you're not going to use them, put those points in Mist, which at least gives you more manueverability at the first level and a useful passive skill at the end.

3) Nightwalkers aren't worth the time: This is only my opinion, and many will probably disagree with me, but I basically see Nightwalkers as nothing but scouts.  I send them off to other lanes to increase my line of sight, but other than that I ignore them.  I never invest in coven/ICA.  I don't view Nightwalkers as effective attackers without heavy investment and micromanagement, and the cost/benefit of making the investment and taking the time to control them intricately is not worth it for me.  For my purposes (scouting), the nightwalkers you get without upgrades are plenty.  As scouts, however, they are quite valuable.  I forget the name of the poster who wrote the guide which includes strategy vis-a-vis Nightwalkers as scouts, but bravo.  It is a very effective tactic and doesn't require a lot of effort, though a sharp eye is often needed (you need to play zoomed pretty far out to see all of some boards.  Again, Erebus' great strength is his manueverability.  Having eyes all over the board is another piece of that puzzle.  On some boards, it doesn't matter much, but on others it can be decisive.  Hopping from one lane to another and surprising an enemy DG who thought he was alone because of one lonely Nightwalker is a very nice manuever.

4) Item "builds" must stay flexible: For me, at least, there is something approaching a perfect skill-tree build for Erebus and I rarely vary from this progression (which I will post below).  Items are a different animal, entirely.  Especially in the early game, it is a very good idea to alter your item build based on which enemies you are facing and what their tactics seem to be.  For example, if I'm up against an UB (or worse, two), I'll invest in a potion/TP scroll/mana pot in addition to armor because I need a bit more staying power to beat off the beast at the initial shock.  I'll probably also take the speed anklet as my favor item.  If I'm up against a slower team (like QoT/Rook or somesuch), I'll just go ahead and buy the first mana helm and not worry about potions/scrolls because the odds of them beating me off with burst damage in the initial push are very low.  A lot of people seem to focus a lot on late-game item builds and go cookie-cutter in the early game, a tactic which I think is very flawed.  Especially if you're facing a strong early-game 1v1 DG like the beast, you need to alter your item build.  This is, in actuality, probably more important than your late-game item strategy, where going cookie-cutter with Mageslayer/Ashkandor/etc. works quite well.  It seems that a lot of players have got the whole item problem backwards.  The game is often won or lost in the first few encounters.

Ok, here's my skill tree build:

1) Bite

2) Mist I

3) Mass Charm I

4) Bite II

5) Bat Swarm I

6) Poisoned Blood I

7) Bite III

8) Mass Charm II

9) Mass Charm III

10) Bite IV

11) Bat Swarm II

12) Mass Charm IV

13) Poisoned Blood II

14) Mist II

15) Muddle/Vampiric Aura (depending on if I'm fighting high-DPS or ability-using DGs)

16) Vampiric Aura/Muddle

17) Mist III

18) Mist IV

19) Bloody Haze

20) Bat Swarm III

The logic behind most of these decisions should either be clear or already explained above.  One small thing I should point out is that at level 12, you basically have a choice between Mass Charm IV and Poisoned Blood II.  If you simply look at the cost/benefit of the skills, you see that going from Mass Charm III to Mass Charm IV gives you 2 extra seconds of stun to creeps and one extra second to DGs, which is quite a lot, actually.  The second level of Poisoned Blood only gives you +5 HPS above Poisoned Blood I (someone correct me if I'm wrong and the +15 and +20 are actually stacking), which makes it actually fairly weak and mainly useful on the way to Vampiric Aura at the end of the tree.

Allright, well, have at it.