Cult of the Lamb

Story-related; cannot be missed.

This content starts by locking off all four regions again, and your new NPC you'll interact with is called ???. It'll open Darkwood up, and you essentially need to progress through the game again, much like a New Game+ run, to access and beat all four Bishops again. The process of completing each region is the same: play through to the end of a run three times, beating a miniboss at the end of each run, and at the end of your fourth run of that region, you'll finally confront the Bishop. The fight is exactly the same, except after beating the Bishop, it'll be there for you to Convert to your cult. Do so and then leave, and then back in your cult you can indoctrinate them. Refer to Relics of the Old Faith (30G) for Bishop quests to ensure you don't miss out on that highly missable achievement.

The other aspect of this achievement to address is actually opening up all regions so that you can play through the story again. This time, each region's door not only requires a set amount of followers, but also a sacrifice of a follower of at least a certain level: Anura requires a level 4 follower, Anchordeep requires a level 7 follower, and Silk Cradle requires a level 10 follower. If you're like me, you likely never really had any followers go beyond level 2 or 3, and you may be wondering how on earth you're going to get a follower to level 10 in a reasonable amount of time and before they die. Thankfully, there's an easy and guaranteed way to do it: the Confession Booth. I believe this was a required build from the base game, but if not, make sure you build it. It's a one-time build, and never gets destroyed. Taking confession from a follower makes them go up nearly an entire level. You can only do this once per day, but followers live for typically 30-40 days, so you have plenty of time. Another daily thing to do is Influence/Intimidate your follower. I'm not sure how many options there are, but in the lower right part of the interaction wheel, you should have an option to do something with the follower to increase faith. For me, it was Inspire, but I have seen others where it's Intimidate. Choosing this option gives a nice little chunk toward the next level too, and it can also be done once per day. So, what you need to do to open the next door to the next region is identify a relatively young follower (mid 30s should probably be your cutoff) and do the following things with that same follower every day:
 

1. Give a Sermon (this gives a little bump in level)
2. Take confession from them (huge increase)
3. Inspire/Intimidate them (nice little chunk)
4. If they haven't gone up in level from those three things, you could give them a small/big gift to give them the last little boost to level up. Definitely do this before taking confession again, otherwise it'll be wasted if they only need a little bit to level up

Once you have a follower of the necessary level, DO NOT use the sacrifice Ritual in your temple. Instead, go to the region's door itself and interact with it. It'll allow you to choose the follower of the appropriate level, and it'll open the door for you.

One final note: this achievement requires all five Bishops, meaning The One Who Waits too. If you spared him and had him join your cult at the end of the base game, there's nothing else to do for this achievement. It'll unlock when Shamura joins you. If you killed him instead, don't worry; this achievement isn't missable. After indoctrinating all four Bishops, return to ??? and he'll give you a blueprint for a statue of The One Who Waits. You'll need to build that statue to add the fifth Bishop to your cult, and that'll unlock this achievement.