In the prologue Kalak says "The enemy will not remain bound by this." Honor also suggests finding a champion to Dalanir saying that the enemy is bound by certain rules. It might be possible that this is what had been done in the past. A pact with champions for each side that bound Odium and prevented him from attacking Honor. Honor being, well, honorable could not imagine Odium breaking that pact while even one Herald remained bound. Odium of course would attack as soon as he was not constrained from doing so, but in this situation I could see Honor not expecting it.