Cult of the Lamb

After your first run back through Darkwood, ??? will open up the portal to Purgatory in the center. Purgatory consists of longer dungeon runs, and here's what you need to do to unlock this achievement. When you first access Purgatory, you'll only have access to one selection: the first run for the default fleece with no bonuses. Go ahead and do this run, and you'll get an idea for how Purgatory works: you need to progress through all four regions in a single run, then beat the boss at the end. I actually found this first run to be one of the harder ones, because you have no fleece, no tarot cards, and no bonuses. Once you get through it, you'll note that you can only access Purgatory once per day, so jump back in on the next day.

When you're back in for the second time, you'll notice that everything is now opened up. There are two screens. The screen you start on is the Dungeons screen, and you can tab over to the Boss Rush screen. On either screen is a display of all of the fleeces, with an icon next to it to start a run. Every fleece on either screen works the same way: there are six runs that need to be completed to finish a fleece's runs, and doing so unlocks this achievement. So, first decide if you'd rather do dungeon runs or boss rush runs. Then, pick the fleece of choice that you prefer. For me, it was always the Fleece of Fates to get four tarot cards at the start of each run. I also did dungeon runs. Remember, you can only do one run per day, so this will take six ingame days. Choose the fleece you want and complete the run. When you come back the next day, you'll see the row for that fleece has a completed icon and a new one next to it. You need to keep completing runs on the same fleece until you've done all six and unlock this achievement.

As far as the runs themselves, they grow in size as you progress across a row, with the final (sixth) run being a full run of each of the four regions, fighting the Bishop at the end of each, and then a fight against He Who Waits at the end of the whole thing. My main piece of advice for these runs is to just avoid combat stages entirely and as much as possible. Top priority should be heart/Forneus stages. If you are missing red hearts, then always go to a heart stage if available. If your red hearts are full, choose a Forneus stage, because one of your options is always some amount of blue hearts where you can add to your existing heart count. In either case (heart stage or Forneus stage), always destroy all of the breakable stuff around. You'll get anywhere from half to two full blue hearts extra from drops. I geared my routes on every run toward heart and Forneus stages, as they can essentially refill your health to max continually. In combat stages, just beeline right for the exit. Don't bother exploring every room. For Relics, pick something to complement your playstyle. For example, I never use Curses, so never bothered with related Relics. Map out your route of each region to go through as few combat stages as possible as a first priority, then prioritize heart/Forneus stages next, and lastly Relic/weapon shops.

Some runs will be very simple compared to others, depending on the luck you get with stage layout and especially what Relic you're given. The dungeons will get more complex with each completed one. The final one, thankfully, guarantees you a heart stage right before The One Who Waits, so make sure you choose that stage to refill your red hearts and add some blue ones. The final boss fight is pretty long, if you don't remember it from the base game: you'll first fight Baal, then Aym, then The One Who Waits, and the a second phase against The One Who Waits. Thankfully, each fight drops a full red heart to help, but you want to manage your health to ensure you can survive the entire ordeal. After beating this final boss at the end of your sixth run on the same fleece, you'll finally unlock this achievement.