Erebus's strength is his ability to escape; he however has, what seem to be, several critical weaknesses, and there is some advice I can give in dealing with him.
He is very mana dependant and requires a great deal of itemization to overcome this, he therefore has mana trouble early game and if you can afford it he doesn’t like the warlords punisher item(I think its to pricey) for these reasons.
The symbol of purity is also useful for removing the movement and armor debuff from bite, and the heart of life can be used for a minimum of 2 full seconds when he enters mist as he can not leave the form instantly. Also, as mist is his only way of farming creeps, other than his low damage melee attack, I recommend taking the armor upgrade from the citadel to slow his leveling down.
Erebus does not like towers, if you are a Rook building a tower farm can make things very complicated for Erebus. If you are any character with a dot (i.e. UB) do not spit right away, focus on attack speed items, force Erebus into mist, and when he leaves it because bite cool down is over, or because he is out of mana, then spit/debuff/whatever, he will not be able to mist again for 7 seconds.
Once you are in mid-game when Erebus reaches level 10 I recommend avoiding him, run/teleport do anything but engage him directly, focus on creep killing and leveling because at level 10 Erebus gets his last level of bite which Steals 900 health, reduces armor by 800, slows by 30%, but this is his most powerful move, so as strong as he is mid-game, that’s about how strong he is going to stay, his bite will stay the same while you continue to gain in level, and when it gets to late game he will no longer have an advantage over you.
Erebus is not OP; no demigod is when their move sets are taken in sum with citadel strategy and teammates.