Well, all in all it could just be a bad match up, it happens rock-paper-scissors style sometimes. I wasn't trying to say you suck (notice all the Maybes?) that was mostly pent up rage from other posts. Sorry.
Most of Rook's direct damage comes from towers which have limited range and no manuverability, or from hammer slam, which is something that can be avoided by moving.
Also, as kokos said, Erebus has two tricks up his sleeve to avoid hammer slam or towers, which are bat swarm (teleport) and mist (go immune to damage, but can't move or attack). Since Hammer Slam has a charge up and is easily identifiable beforehand, it;s easiest just hit "2" on my keyboard and go invincible making you waste mana and do no damage to me.
Combined with the fact that you are an already slow character, bite reduces your speed, which makes it harder for you to get away. Also, you have high armor, but bite also reduces that amount as well.
So, after factoring all that in, essentially we have:
Rook, who has normal armor (when bitten) and super slow speed (when bitten) fighting Lord Erebus who he can't do any nuke damage to (Hammer Slam missing) and Rook can't run away because he's slowed from bite and Lord Erebus has a teleport that damages to boot.
Just a bad, bad matchup perhaps. Lord Erebus engages you, and essentially takes away your ability to drop hammer slam, ability to run away, drains your health to him, and lowers your armor.
Only thing I guess I can really suggest is play 1v1 against a friend trying different build or items till you find something that works.