Dark sword, light sanctuary

The detailed explanation of the light sanctuary doesn't make sense. It says to gain more ally pieces, level it up, well at level 7, it hasn't gone up a single point, and at roughly 8 available allies to try and unlock with still only having 3-5 per run, is like trying to unlock a god tier weapon at level 1, it's not going to happen. There isn't anything in the official guides about it, and like i said, the description doesn't help at all. Any clarification would be helpful, thanks. Sage