Relics of the Old Faith is basically a NG+ mode without being a NG+ mode. You use your existing save so you keep all your cult progress, but the dungeon areas reset allowing you to play them again on a harder difficult with some adapted boss fights. It's nothing revolutionary, it's not going to wildly change the core loop of the game, and the random new random reward system kind of sucks, but the various new modes, new items, relics, and new cult buildings are all solid additions to an already great game. And the best part is - it's all free!