Are there any rules for using a bretonnian warband in Mordheim?

This should be in the other games ection and no I do not think there are.

There are no actual rules for Bretonnia in mordheim, but if you look on the internet you can find unofficial lists that people have created for those who would like to use Brets in mordheim.

One such list can be found here http://www.specialist-games.com/mordheim/a...nianWarband.pdf