Mist seems to be, besides Morale, the most maligned of Erebus' skills. Some people seem to appreciate the value of Mist I for the manueverability if provides (getting rid of debuffs, avoiding slow attacks, etc.), but I find myself using Mist more and more when I play, and I thought I'd share my success with this skill. Why I use Mist:
1) Survivability: See above. In my opinion, every Erebus build should buy Mist I in the first three levels for the survivability it provides. Especially if you're dealing with heavy debuff or DoT opponents (like a spitting UB), going into and out of Mist is hugely advantageous.
2) Creep farming: Early game, one of Erebus' biggest weaknesses is his lack of AoE and his inability to farm creeps efficiently. A simple Mist I for the minimum duration will hit both minotaurs and archers for the necessary 25% of HP to get full experience/gold for the kills. Early game, I buy Mist I at level 2 and use it as an opening manuever when I engage a wave of creeps. Typically, I time it so that I arrive just behind my creep wave, and walk through the enemy wave to a point just between the minotaurs and the archers. I engage Mist, and then deactivate it as soon as possible, followed by an attack on the archers. The radius of Mist is big enough to hurt all the enemy creeps and heal my minotaurs, who will finish off the enemy minotaurs while I am killing the archers. This is the only fast way Erebus has of clearing creeps in the opening, and it's not actually that expensive in terms of mana, if done correctly.
3) Tower Attacking: See above...my creep wave then "wins" with the help of Mist and generally pushes onto a tower. I can follow behind it and attack the tower for as long as the creeps are alive to draw fire and then retreat before the last one dies. This is the only effective tower-pushing tactic that I have discovered for Erebus in the early game. It's still not spectacular, but it allows EB to at least make headway against enemy towers early on without getting badly hurt. I have been able to consistently take down a tower or two if left alone in a lane when using this strategy, which is a lot more than I can say for most EB players. Communication with teammates is key here in a competitive game, as EB can often hurt but not finish off enemy structures. Learning the correct timing of when to retreat (as the last creeps are dying) is quite important. Mana potions are obviously useful, as well.
4) Creep Farming Part II/Minion Killing: It is a common misconception (one which I have also held) that Mist scales poorly into the mid and endgame. First, the mana costs never increase, so the longer the game goes on, the less Mist ends up costing you relative to your mana pool. Calculating the DPS of Mist as it scales through the game, it is only slightly behind the infamous Spit, going from 50 to 75 to 100 and finally to 125. Enemy DGs who elect to stay in a Mist cloud are going to suffer real damage. Also, Mist's damage scaling makes it effective against minions and upgraded grunts (priests, etc.) through the midgame. Erebus can struggle against well-managed heavy minion-build generals, but I have found that the combination of a well-aimed Bat Swarm + Mist directly in the middle of the crowd as an opener against generals who like to surround themselves with minions can be very, very effective, especially if it's a team fight and the opposing general is too tied down for the micromanagement necessary to save his minions.
5) Hiding: This is a big one, that I quite often use to my advantage. Erebus is among the smallest of the DGs, and the only small one that you're ever likely to see in the middle of a big scrum (that kind of bravery from TB and Reg is generally not seen). When you get to midgame and the screen starts to get full of nonsense, Erebus' small sprite can be difficult to target when he's in the middle of a crowd. He is especially difficult to reacquire after he comes back from Mist. I first noticed this effect against an enemy EB, and found myself mistargeting him and instead attacking enemy creeps because I simply couldn't find his sprite in the middle of the chaos. In my own experience, human opponents quite often need a couple of seconds to reacquire me and attack if I mist/unmist in the middle of a big crowd, at least in part because they don't know when I'll unmist. This works even if you have killed the enemy creeps/minions, as their icons stay on the screen for a couple of seconds in their death animation, though if you intend to kill them outright with mist, watching their health bars and timing your unmist just after they die is key to making good on this tactic. This is clearly a team-oriented strategy (going into large scrums alone against multiple DGs is almost always suicide), but one I find extremely useful. And, after all, Demigod is a team game.
6) Endgame: In the somewhat rare event of a nearly stalemated endgame in which CpSa and Giants are bought by both teams at nearly the same time, Erebus (and many other DGs) can often get lost in the chaos of all that meat. Mist (theoretically upgraded to Bloody Haze by this point) can be a real difference maker at this point because the creeps are now the most important figures in the game. With Bloody Haze enabled, your CpSas and Giants are regenerating 155 HP/second, making them very, very difficult to kill, even by creep farmers like TB - and 125 HP/sec is done to enemy creeps if two waves meet or to enemy DGs if they decide to move into the mist. Escaping from the middle of a pack of giants is quite easy because Erebus is nearly impossible to see in this case (see above) and regenerating the heavy mana this costs with the HoL is no big deal. Mist/Bloody Haze can be a real difference maker in a tight endgame when the big units come out.
7) Better than Mass Charm: I find myself using Mass Charm less and less. It is quite mana heavy, and largely useless from a cost/benefit analysis until it reaches level 4/Muddle. An ability with a casting time of 1 second that stuns enemy DGs for anything less than 3 seconds is garbage, and I don't really care about stunning creeps because Mist deals with them much better, anyway. Mass Charm is my late game investment - what I level up with my last five levels if it comes to that, which it rarely does. Early game, it is terrible, and even midgame, unless you're skimping on Bite, Bat Swarm or Poisoned Blood and have bought zero levels of Mist, you're not realistically going to buy it up to the point of usefulness until level 12/13. It's a poor investment of skill points until you get into the late game.
At this point, my preferred skill tree looks like this:
1) Bite I
2) Mist I
3) save
4) Bite II
5) Bat Swarm I / Poisoned Blood I
6) Mist II
7) Bist III
8) Mist III
9) save
10) Bite IV / Bat Swarm II
11) Poisoned Blood II
12) Mist IV
13) Mass Charm I
14) save
15) Bloody Haze / Vampiric Aura
16) Mass Charm II
17) Mass Charm III
18) Mass Charm IV
19) Muddle
20 Bat Swarm III
Obviously, it's an assassin build, although I should note that I also make use of the Idols in the midgame. I have a generally low opinion of the Nightwalkers, especially against skilled opponents, and EB's other skills are simply more valuable. I also haven't noticed that the Bite nerf has slowed me down much.
