A new addition has been made to faction hordes. You gain potions at certain levels of the horde. A Potion of Power is gained at horde level 200 and with additionals gained every 25 levels after that. The horde is built up using treasure cards and gold. I can confirm it does not have to be the potion at 200. I had one at 299 and added just enough cards to kick it to 300. Achievement popped shortly thereafter.
Playerforty4 notes in the comments below if you are already above level 200, you only need to raise your horde one level. This meshes with what I did to get the achievement to pop. Another player says you only have to add one more treasure. That I can't confirm but it usually doesn't take too many treasures to up the horde a level so it's worth a try.