THIS WEAKAURA NO LONGER WORKS STOP ASKING ITS DEAD
THIS IS NOT MY AURA JUST A PORT OVER FROM THE CHINESE VERSION WITH A MORE "CLEAR" EXPLANATION FOR THE PLEBIANS
USE AT OWN RISK, THIS SHOULD BE A PROTECTED FUNCTION AND SHOULDN'T WORK BUT DOES. MAY CAUSE UI TAINT AND BREAK YOUR UI.
Video of it doing the evil three swords flow thing, you can see it happen here 
on the UI and it swaps around 7s and 45s into the highlight - note that this does work in all settings you just have to hit the conditional events for it to swap back to your Black Magic wep (<1 rune & <40 RP - this assumes you are on a dead global and swapping the weapon on won't incur a loss of gcd).
1. Import Aura
2. Enable Equipment Manager here:
3. Create Equipment sets;
One with your normal weapon (with Fallen Crusader or Berserking).
One with your Black Magic weapon
4. Place these sets on your MAIN ACTION BAR (1, 2, 3, 4, 5, 6, 7, 8, 9 , 0 (10), - (11) and = (12)) - be careful if you are using ElvUI or some other action bar addon as what you think is your main bar may not actually be your main bar. BY DEFAULT this is set up to have your BLACK MAGIC SET in slot 9 and your NORMAL WEAPON SET in slot 10. If you have these keybound to other things, it does not matter.
5. If you want to change the slot on your action bars open Weakauras menu and navigate to the CONDITIONS tab. CONDITION 3 is where you want your NORMAL WEAPON SET, change the code to whatever number you want it to be on. CONDITION 4 is where you want your BLACK MAGIC SET, change the code to whatever number you want it to be on.