Certain models can hire other models out of faction. For Example the Dual Faction Jacob Lynch can take up to 4 DARKENED models out of Faction.
IE if he is playing as Ten Thunders but wants a Beckoner and a Depleted, as they are both DARKENED he is able to take them even though they come from a different faction. I believe MIMIC is related to Lucius.
The rules concerning these are normally on the Masters card or one of there upgrades.
Hope that helps you out a bit.
(And Ninja'd by ZFIEND! :-) )