Erebus dominates early on(Bite is very effecient)
Is it a joke? At the begining you're lacking mana and bat teleportation and you can't tank. Even the mist is useless (edit:I mean for kill another dg and compared to a heal/shield skill at same level) until you get enought mana item for support it (its just a debuff/attack evade, like fireball/snipe/hammer). I find erebus one of the weaker at beginning. And if you play vs a sedna or queen (the two it's worse) you can do nothing. In 1vs1 in the first levels, it's impossible for erebus to win against thoses two, and even in a 3vs2 battle on the map, the heal/shield at the beginning if well played help Sedna/queen and a teammate to stay alive. Against a rook playing def with tower it's same. Well played at begining I dont try to charge it, even when I get first bat teleportation, or only when it's 3vs1.
I find that Erebus get all his potential only when you get the 4 skills and enought mana for spam them. At this point he become a killer and later level 15+ is the best. But until you get all the item/skill, well, you can already loose the game. Of course I've never tried Erebus with full creeps. Wonder how it play. Anyone can talk about it?
EDIT: About skill cooldown after mist, thought it was the point to be able to undo mist fast and attack just after, at the moment other players forget about you and you're 'deadly' bite. Used it 1-2 times like that, transform into mist waiting teammate coming help me, and during the battle that occur over me come back and finish the one trying to escape.
If you want stun/silence erebus when he stop mist and use bat, dont stand in middle of the mist. Just camp the path far enough to stuck erebus after a teleportation. And still, you need enought mana for a teleporation after a mist, it's possible only later in game. Before that, even if you can do it, you are out of mana after the jump and can't bite/mist again...
LAST EDIT: Rook towers are a good counter vs mist. IF you go into mist for life save an have low life, just when you go back normal the tower hit you. You don't have enought time for teleportation or escape and mist don't damage the towers (didn't noticed it). Fire ring can be deadly too when you don't have enought mana for stay in mist. The ice rain can caught you after the teleporation if the TB is well placed I think.