max - you asked.
Arithon had wardings up - and the attack by the Mistwraith cut through - for that short period before Asandir showed up, it was 'inside' his defenses - he was fighting for his life, on an energetic level - the wraiths can feed on life force. When they do, they get stronger.
Diffused in the fog, over all of Athera, the effect was also diffused (there were not enough wraiths to choke a whole planet, Traithe cut off South Gate before enough got through - Athera's infestation was never as deep as Marak's). As the fog was reduced, the wraiths were concentrated into the area over Ithamon. You will recall, that in this attack, the wraiths "tipped their hand" - displayed traits not seen by the other members of the Fellowship.
There is a greater thread at work here - I can't tell you any more at this stage of the story's development - you'll have to read and see.