In beta, I found you HAD to use mist form because Erebus was so weak otherwise he would die in most fights toe to toe. Mist form let him prolong/hide. It now costs more mana to do, so it's harder to use it as a way to prolong a fight. It can still be a good way to avoid a fight. I don't remember what max level mist was... but I know the weak one did 25. Anyway I think the key to mist form is not just the AOE damage + immunity, but that it also heals your minions/allies.
Currently I find his health regen to be really really high. Combine that with the life gain of his bite and I can get him so he just tanks quite a few fights and because it's hard to run away from him, you kill people easily.
Most games unless you're fighting equal level opponnets or just screwing around will end before level 15.
My minion build is very very strong and if you've not found that build, not sure I'd be making so many balance suggestions [e digicons]:blush:[/e]
I typically only put 1 point into bite, 1 point in mass charm, 1 point in Bat form, and 1 point in poision blood. I don't take mist at all. All the rest of his points go into moral, and his two other minion abilities but you also pick up level two poision blood when it's avaliable. By level 10-12 I'm a monster!
This might be because his minions do too much damage with moral, so it might need a slight nerf, or it might just be that he can tank damage too well, hence my suggestion on nerfing the health regen.
+10 and +20 health regen is crazy good imo. The poision potion is just gravey on the side.