[quote=mattmanganon]As for the Faction of Chaos... I was thinking of either Word Bearers. Thousand Sons, or Alpha Legions... Any preferences?
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I think Word Bearers in this case would make an good oppenent. They use all sorts of different daemons (Thousand Sons only use Tzeentch and Alpha Legion doesn't use Chaos at all) and they also try to convert people to their side. This can become a critical part for both female character. One trying to figure out which of the three doctrines are the strongest (Tau, Imperium or Chaos) while the other represent what the Word Bearers once were and what she could possible become if she falls to chaos.

Edit: Also how much do we know about the missions? Do we know about who we are fighting (chaos) and not fighting (tau).