Chalice dungeons are a big part of the game, or at least they worked on it a ton. Not really to do with multiplayer, though you can do it, but it adds on a ton of play time. Nothing to do with the main story and ng+ or whatever doesn't change them. You go to one of the stones, do the chalice ritual with it and it's either a random area with random bosses if you have the root chalice, or if it's the normal chalice then it's pre designed areas. The first chalice is really easy once you level up as it's the starting one, and there's like 3 layers with a boss at the end of it and random loot throughout. You can get special types of weapons too. Like there's 3 types of each weapon, and there's always a bonus room, and once you clear all of that you can get random loot and sometimes you can get like a "lost blades of mercy" instead of the normal blades of mercy, the difference being the blood gem allocations I think, and you can get a blood rock as there's only 1 in the normal game. It's a lot of fun though, and they get really hard as you go on.