Anti-Climax Boss: For all the buildup he gets, the Leviathan is incredibly easy: its attacks, while admittedly damaging, are all telegraphed a mile away and slow as molasses, making it almost impossible to be caught in a position where you can't react in time. Equip an offering that lets you dodge underground like the Mole Claw, and taking advantage of its invincibility frames leaves the Leviathan utterly unable to hurt you. From there, it's just a matter of whittling down its admittedly large health pool, but considering how its bite attack leaves it open for a good 7 or 8 seconds, that's plenty of time to wail on its cursed spots, which take up almost its whole head. Add to that the fact it has an elemental weakness you can exploit, and even its health pool goes down startlingly fast.
Anvilicious: Delta's ending. The player character just will not shut up about how much better mortals are than Gods, to the point of undercutting Librom and Magusar's importance.
Fridge Horror: If you save Magusar in the final battle, the protagonist begins a cycle of defeating and saving Magusar over and over, while line between the protagonist and Librom becomes blurred and they become one. They fight with Magusar for years until the protagonist's body breaks down and can no longer fight. Like the original Librom, they sacrifice their body to become a book that will pass on both their story and Librom's tale to the next prisoner that can take on Magusar. While scary enough on its own, the cutscene shows SEVERAL books there, making one wonder just how many people have done this...
A milder example, but some of the sufficiently powerful Stonefist spells, particularly the breakable ones from Musicians of Bremen or Red Riding Hood in Delta. While they do have a short windup, you can use them to completely pummel Archfiends who are downed, stunned, or simply not paying attention to you and pile up truly mindblowing amounts of damage. For reference, the gold edition of Red Riding Hood's attack, which clocks in at over 400 damage per use, and hits multiple times in the right position. Add some damage sigils and a Dark Arm, and you're good to go.
Delta makes blood magic very useful by introducing new melee offerings. They're very spammable, capable of earning Counter bonuses, and do not require a summoning animation like the other melee offerings (arms, spears, and swords). Certain Archfiends can drop better melee blood offerings than the basic version, and at earlier points of the game (once Carnatux's debt pacts are available, a skilled player can get one about seven missions in). As for blood offerings' biggest drawback, the "Damage dealt"/"HP drain" ratio is more efficient than the blood shot offering. In case a player still has difficulty with carrying strong enough healing magic, all allies are guaranteed to carry a healing offering. After saving enough sorcerers, a player should hopefully collect enough allies with area healing spells to support a blood build. The best Blood Offering to use is the Chthonian Claws dropped by Chthonian Fiend, as that Offering has a massive base damage of 336 when the Large version is upgraded to Black, and it shreds through bosses and mooks like Swiss cheese, regardless of elemental affinity.
A divine arm build has the potential to be what can only be called The Juggernaut. By combining the right arm and/or heart sigils, specifically the ones that boost damage, you can more than make up for the divine arm's lack of base attack with modifiers reaching into the 500%'s. Add that onto the divine arm's natural bulk, and you get a near-unkillable Lightning Bruiser.
The Berserker Black Rite is easily one of the best Black Rites in multiplayer, as it staggers the Archfiend you're fighting over and over for a very long duration, letting your allies whale on it with impunity. And the cost is fairly trivial (unable to discern what offerings are on what button) if you've memorized which buttons to press for your offerings.
Player Punch: A divine arm playthrough in itself. Your organization hates you because you're saving monsters instead of eliminating them, as they want, then they send bounty hunters to kill you, and you cannot save them, you need to sacrifice them to keep going. Then you're going to endure joining parties where most players have dark arms, and they'll want to sacrifice the very monsters you intend to save.
Averted in Delta, which does away with the assassins and adds a divine faction in Sanctuarum. In fact, a quick check of the rankings in Delta puts Sanctuarum in second place, behind Grim (the canon ending), making Avalon dead last.
Calixto's narcissism is utterly grating, and dealing with him is a pain in the neck. Then you find out the truth behind how it all started, and you will hate yourself for being so hard on him. Remember all those times the Player Character insulted him and called him a 'vainglorious cad', and Calixto recoiled like he got slapped in the face? Yeah, that was his wife recoiling in horror as she hears you insult her husband from within his right arm. How did she get there? Well, she used to be a monster, and even though Calixto saved her, she was haunted by the notion she could regress, and ultimately asked him to sacrifice her because she couldn't bear living anymore. He does, and by absorbing her love for him, he gained his narcissism.
When starting the game, you may decide to save every enemy you beat, regardless of Sortiara's complaints. Then you reach the end of the Ordeal. You'll probably try to save Sortiara, which quickly teaches you that saving enemy sorcerers is a bad idea. She's the first enemy you're forced to sacrifice. And it isn't very fun.
Sequel Difficulty Drop: A Crossover rather than an actual sequel, but this game's version of Dionaea is much more forgiving than its home universe counterpart. For starters, the original Dionaea is a Lightning Bruiser with broken hitboxes, an absurdly fast and difficult-to-dodge charge attack that Dionaea loves to spam, fast fireballs with nearly perfect accuracy, dangerous winds, and a massive Area of Effect that eats up 85% (if not, all) of your health while firing its Wave-Motion Gun. This game mercifully tones this Abductor down to manageable levels, now that Dionaea becomes a Mighty Glacier with sensible hitboxes, a slower charge attack, slower and less accurate fireballs, harmless but still annoying winds, and a weaker and smaller Area of Effect attack.
Tear Jerker: The Minotaur's backstory. All she ever wanted was a family...
Really, a lot of the archfiends' backstories could count, as while some of them deserved what was coming to them, the other archfiends really didn't deserve such a cruel transformation.
The death of Similia in Delta.
That One Boss: Pretty much all of the bosses have at least something that makes them a pain in their own special way, but there are a couple that just go above and beyond.
Beelzebub: you will learn to hate flies, particularly when their master keeps pelting you with them, and has a nasty habit of generating a harmful shield around himself to deter physical attackers.
Behemoth will relentlessly spam homing tornadoes over and over and over again, making it incredibly difficult to land a hit on him as you're dodging constantly. Once he drops a couple of his apple drones, the whole affair can quickly become a frantic mess.
The Iron Virgin. Good god. She's incredibly fast, her attacks are unpredictable and very difficult to avoid, said attacks also hit extremely hard, can power herself up like the Dullahan to turn her attacks into borderline One-Hit Kill, and is also annoyingly far more durable than an archfiend of her caliber should really be. Seriously, fighting her takes forever, even moreso in Delta, where they removed her weakness to Fire. Even worse? She's the boss of the Peninsula of Power Leveling, so expect all your hard work Level Grinding to go down the drain if you lose to her.
There are reasons why fans like to call Cat Sith "Cat Shit''. He's a relentless, annoying Archfiend that hops all over the place, turns invisible unless you use Mind's Eye, has attacks that are hard to dodge, and spams giant cats that deal so much damage they can even cripple a Sanctuarium player.
Woolseyism: The Arthurian mythos references have been veiled in the US and EU releases due to some areas of Europe still treating the mythos as Serious Business, hence Nimue's name change to Sortiara and Merlin's to Magusar.
, Tearful Tone