You must go to Charlotte, John and Paul to make them break in Arwin's cellar. This part is straightforward. If you are exceptionally fast, it may still be possible for Charlotte to be detained though. She must be out of jail to join.
Finding a merchant
Two merchants can potentially accompany Aura to the festival: Hermann and Edwin.
Talk to Hermann after your first date with him, and he will propose you a second date in exchange for a variable monetary reward (0/200/400/800 G) and his invitation to the festival.
If you have completed Papertrail and invested 2000 G in Edwin's activity, he will owe you a favor. This favor can be converted into 3000 G if you chose Hermann to accompany you to the festival. Else, you can use the favor to get Edwin to accompany you.
Once you found the merchant to accompany you, talk to said merchant to go to the festival. This will end the day and you will enter the festival the next day.
Some advice before going to the festival:
Having at least 16 AGI will allow you to avoid a battle where Aura fights solo.
Having a rope will allow you to fight a missable mini boss for a Scorpio Claw and harvest a missable pyromantium.
Having a vitality potion in your inventory will allow you to get a missable reward after the quest if you unlocked Vice.
Once you're in Arwin's Mansion, you may choose to apply Coatings to Aura such as Nasty Oil Coating and Poison Coating and they will last for the duration of the quest. This can be extremely beneficial at all stages.
The guests
You start by entering Arwin's mansion. To progress further, talk to the following guests:
the man with a big sword in front of Barry,
the woman in fancy clothes,
the man in front of the white priestess,
the adventurer with blue hair,
the strange individual arriving when you talked to enough people.
After this, go to the junction point between the dining wing room and the entrance room. You can now access Arwin's cellar.
Arwin's cellar
At this point Aura is alone. Getting defeated in battle will end the day. The next day will bring you on the mansion's first floor.
Your goal in to reach a lever allowing your comrades to enter. The direct path to the lever will lead to two battles. Both are avoidable.
The first battle can be avoided by taking the underground waterway and destroying a rock. You exit this path behind the enemy which could have spotted you. Along this path, you also find a rope point leading to a mini boss fight, but it is better to wait for Aura to get her three partners before trying to fight. Alternatively, you can avoid the first battle by luring the enemy, going around the table behind Aura she she is followed, then entering the fissured wall.
The second battle can be avoided by taking a jump point. Note that winning this battle yields a key. Getting this key allows you to open a future door without wasting a bomb or removing Aura's collar, but it is better to get Aura's partners before fighting.
Reaching Arwin's domain
Once Charlotte, John and Paul join your party, you can go north to continue the quest. After opening the door leading to the last part of the cellar, there are some hard to spot chests you can loot.
Further is a battle against a mini boss: the slime summoner worshipper. This enemy summons a small slime on his first turn, a fire or earth slime on his second turn and the last enemy on his third turn. The summoner has 450 HP, 21 DEF and 27 MDEF. Defeating him before he summons three enemies is key to make the battle manageable. So either remove Aura's collar, or use Charlotte to boost your party's ATK while hitting the summoner. Given the small slime will likely use Protect I on turn 2, use this turn to get tactical advantage.
After defeating the boss, continue to discover a domain. There is also a chest containing an energy potion, so the domain can be opened even if you ran out of MP.
Before entering the domain, one of the merchants in the room can be interacted with. It turns out he's not dead yet and can be saved if you have a vitality potion. If you save him, you can find him after the quest in the Congregation building. There, a Vice action can force him to give you 1000 G.
Reaching Arwin
Inside Arwin's domain, you will get a curse from Mammon. If you choose to remove Aura's collar when prompted, you get a weaker version of the curse. This will also restore Aura's stats to their maximum. You can choose to spend 6000 Perika to re-enable collar removal. You also get a Perika counter indicating you have -1000 Perika. Finally, you get the sidequest The Price Of Freedom.
The gatekeeper blocking the path to Arwin will only let you pass once you have the VIP pass. You need 20000 Perika to buy this pass. Here are all the ways to gain Perika:
You can use liquidation money spells. They are under the Special category and can be used outside battle.
You can convert gold into Perika at a stall.
Close to Desmond's location, there is a maid attending to a guest. Talk to her and accept her lewd offer (3 base minimum lewdness) for 8000 Perika.
Close to Desmond's location, there is a small hole. Entering the hole makes Aura temporarily gather her three companions and enter a zone where fragments can be fought. This zone contains 7 chests. Each chest contains 1000 Perika. The exit point is close to a lever. If you turn the lever on the right, the blackjack game near the exit becomes rigged in your favor.
There are 4 blackjack tables west of the domain's entry. Losing makes you lose 500 Perika. Wining yields 2000 Perika. If you won at a table, you can't play again. By default, the game is rigged so that you always lose. But you can rig the game so that you always win (see above).
Northeast of the domain's entry, there is a clown taking bets on the human derby. You lose 2500 Perika if you lose. Wining yields 7500 Perika. You only get one bet. If you already freed the three participants, the derby won't happen and you can't bet. An option is to free two of the participants before the race begins though. If you bet on a race with only one participant, you are sure to win. If you take this option, free the male abductee and the blonde abductee (costs 3000 Perika in total) to save Parika. Doing so makes you miss a reward (see last section) If you want to spend even less Perika, free only one abductee, or none at all. Here is a table giving the most likely winner depending on who you freed before the race:
Freed
Likely winner
Explanation
No one
Blonde abductee
The male abductee is most likely to start first.
If not, the blue haired abductee will win.
Blue haired abductee
Blonde abductee
The male abductee is slower and will get pushed.
Male abductee
Blue haired abductee
The blonde abductee doesn't get first on the rope.
Blonde abductee
Male abductee
The blue haired abductee won't push someone else.
You can open a golden chest near the stalls. This chest contains 10000 Perika and can be forced open (requires to remove collar, losing 200 HP or having 30 ATK) or opened with the combination (an NPC watching the human derby outright tells you Arwin's favorite number is 1).
Northwest of the domain's entry, ther is a small arena where you can fight a duel. John will fight at your side. You can talk to him to heal both of you (requires 200 Perika). Losing makes you lose 1000 Perika. Wining yields 10000 Perika. You can repeat the fight after losing, but not after wining.
Once you show the VIP pass to the gatekeeper, Aura's HP gets reduced to 1 and you can't get back before the gate. So if you want to buy some items before fightning Arwin, do it before showing the VIP pass. Having high Perika is also useful for the upcoming fight.
Once you are ready, follow the path behind the gatekeeper to fight Arwin. Before the fight, you receive a willpower malus if you did maid work at least one time. If you worked two times, the willpower malus amounts to 30.
Fighting Arwin
You fight Arwin and Mammon. You are backed by Aura's three companions.
Arwin can spend Perika to: summon a small slime (he always does this on turn 1 and does it again regularly if there is no slime in his party), buff his party's attack, take flight (only on low HP), heal himself to full health (only on low HP), and create 3 bombs (only when he is flying). When he doesn't spend Perika, he guards. Arwin has 500 HP and low defenses. So, when attacking him, you should dispatch him quickly and not give him an occasion to take flight. Alternatively, if you bought a web bomb, you can use it to bring him back to the ground.
Mammon is a damage dealer. She can use Superbia (and she always use this skill on turn 1), normal attacks, Cupiditas, Avaritia and Slash II (only when she has tactical advantage). She's tougher than Arwin and can only take significant damage from boosted offense or using Pierce I.
When you win the fight, Arwin and Mammon come back to life. The fight starts again. In total, you must win three or four consecutive fights against Arwin + Mammon. Here are some options to win the fights:
Removing Aura's collar and using the Purification money spell to make Remove Collar available for the next fight. Having 10000 Perika and Remove Collar available before facing Arwin is enough.
Heal, prepare Tailwind I and Water Skin I before facing Arwin. Invest in concentration with Charlotte, Counter III with John, and ATK up with Paul and Aura. Using MATK up twice also lets Charlotte do high damage every turn. Use John to protect his teammates while the other party members go on the offensive. John and Paul can get tactical advantage with stasis bombs. At the beginning of each subsequent fight, use Paul to heal up on turn 1. Mammon's slash being very dangerous, you should defeat her quickly/use stasis bomb as a counter/bet on John's protection coupled with his counter III state.
After Arwin runs out of life insurance, he takes the confontation to another level.
Fight by proxy
After reading the rules for Arwin's new challenge, you must choose a party member to participate in a combat round. You can heal up between each round. Here are some advice:
Begin with making Charlotte fight for 4 turns and get on full offense.
If Arwin sends a monster close to be defeated, send Paul or Charlotte.
If it is possible to get a fight Charlotte can't win, send John.
After defeating Arwin in his game, you gain 10000 Perika. After that, you'll get a scene and Aura will remove her collar. Aura automatically use her Purification money spell if removing her collar is not immediately possible.
Mammon's core
You fight Mammon at her full power, but you are using the full Star Knightess abilities, so the fight is actually easy. If you struggle to win, follow those steps:
Spend Perika to reduce your Corruption (Perika will become useless after the fight anyway).
Use Step of the Rat and Power of the Horse.
Get tactical advantage.
One shot Mammon.
The quest is now complete.
Bonus rewards
Depending on how much time you took to complete the quest, some NPCs will give you additional rewards.
If you complete the quest before day 20, one of Arwin's maids can be found in the refugee camp. She gives you 2 vitality potions.
If you paid in full the most expensive debt at the human derby, that of the blue-haired girl, and instead of leaving her as the sole participant, she can be found in the refugee camp. She rewards you with 3 emerald teas.
If you complete the quest before day 30, a merchant in the congregation gives you 500g.
If you complete the quest after day 19 and before day 40, one of Arwin's maids can be found in the refugee camp. She gives you 1 vitality potion.
If you complete the quest after day 29 and before day 50, a merchant in the congregation gives you 200g.