Once a challenge is accepted, the character who accepted the challenge is moved in the ranks so that the two protagonists are opposite each other. If the models are especially large it may be more convenient to remove them altogether and place them beside the fighting units.
When combat is worked out, these two will fight together. No other models may attack them even if their bases are touching, and no other models may shoot at them or use magic to attack them.
Once the challenge is underway no further challenges may be issued in that combat until one character is slain. The challenge might therefore last over several turns of combat.
If characters are riding monsters or steeds which have their own attacks then these fight during the challenge as well as their rider. If the character is riding in a chariot then the crew and creatures do not fight in the challenge at all (see chariot rules).