Here are a few key points to remember about Black Betweens.
* The BB spell is actually SIX spells, one for every Region Type.
* Once you cast a BB Spell on the appropriate region, you cannot do so again while that BB remains under your control.
The territory that you've cast it upon also stops generating any Treasure.
* By casting a BB, you automatically upgrade your lessers one step, This upgrade happens at the end of the Turn after all battles have been fought.
* Remember that once you cast a BB, you automatically upgrade your Crusaders to Amra's who then have an easier time clearing out Natives (who are Crusaders)
The sooner you can get your hands on a Plains BB, the better off your forces will be against not only Natives, but other players who have yet to cast one.
* Cast the 1st Black Between on a Plains Region that you can protect easily, so by default, usually in the heart of your Empire.
* Black betweens get progressively more expensive, starting at 125 fury for a plains BB through to 4000 for a Mtn BB
Check out this link for a more detailed description of The Black Between.
http://wyrmmlore.com/evernight/newlounge/spelledout.html#Black