My Dark Souls D&D Campaign Was a Great Study in Dungeon, Encounter, and Monster Design

I have a strange love/hate for Dark Souls as I've always found the gameplay very fun, but many other elements I found to be inconsequential to the overall experience.  I have a very gameplay focused group for one of my regular sessions, so I felt that Dark Souls (the first one) would be an excellent thing to use as a basis (well to copy almost completely) for a very gameplay heavy campaign.  The world of Lordran is just one really, really big dungeon, though you need to play the game to actually understand how to integrate it.

There are some very obvious points where this setting needs work- you will need to provide answers for questions the games and their lore does not provide, expand the motivations and attitudes for characters where there are none, and prepare for situations that the game can't handle.  This actually complemented my strengths as a more roleplay and world focused DM as it allowed me the time to make the those choices when preparing for a session and let the already existing gameplay elements stand as they are.  The game provides a basis for how encounters should play out- scary, moment to moment choices, close calls, and sudden death abound.  Design can be muddied when worrying about what something does before how it should feel, provided players know what to do. (tangent: Unkillable enemies and unavoidable effects for drama are almost always bad design)

These are the gameplay changes I implemented to keep the theming of Dark Souls souls intact and to keep every encounter balanced.  Dark Souls is a game about skills and reactions, while D&D is a game about imagination and luck. There are no extra actions for rolling or parrying-just use those as flavor for missing or critical hits.

• DO NOT tell your players it is a Dark Souls campaign.  Let them build a character designed for gameplay and make it clear that backstory choices won't influence the game but active roleplay choices will.
• When you reach 0 hit points, YOU DIE....then you respawn at a bonfire (maybe).
• Simple but Impactful enemies- high armor class and health that scales with the number of player characters- but with noticeably exploitably vulnerabilities. (For example, skeletons weak to bludgeoning and radiant, but with strong dexterity, and a shield)
• Bonfires have a lot of rules- they function like they do in the games and allow long rests- but importantly they have an increasing chance (+5- or 10% to go out each time they are used to rest, to impact diificulty)
• Spending a resource, called "Humanity", is used to respawn/revive players.   You can also use them to get the benefits of a short rest in an emergency.  Like the games, you use it to "restore humanity" to give you that extra life, or just to revive an unfortunate party member who died without it.  No Humanity  TPK = Campaign Over.
• Limited resources.  Start with basic armor,  shields, and weapons- no heavy weapons, and no ranged weapons or ammo (except ranger, yay!)
• Nerfing magic and ranged attacks:  Ammo tracking, 5 level 0 spell slots for casters(for cantrips and some normally unlimited class effects), and full casters obtain spells at the rate of half casters while maintaining slots (second level spells at level 5, etc.).  Progressing in the campaign has ways to get around these restrictions.  Also, I've not dealt with it yet, but lingering spells like flaming sphere and spiritual weapon would change to either instantaneous effects or some kind of melee buff.
• New condtions:  Stagger- getting hit by an enemy causes all other enemies to have advantage on attacks.  Heavy Stagger: Certain heavy strikes cause a Con save against incapcitiation.  Once on hit, and another at the start of your turn if you failed. (This was the first way I could figure out how to make barbarians and moon druids not completely busted). Players can stagger enemies too.
• Bleed:  Certain weapons cause you to take bleed damage when you are reduced to or below half health- you roll your highest hit die and lose that much health  (this was the second way I could figure out how to make barbarians and druids not completely busted).
• Cleave: Big enemies target multiple creatures within thier reach with attacks, often at the expense of heavy stagger.
• Combined gold and experience into "Souls" just like the game.  Easier to track, and there is plenty to acquire.  I didn't even change the amounts from the games (doing that for every enemy sucks).
• Same as the games, dying leaves a point you have to reach to get your souls back when you die.  Die again and they are gone for good.  Oh yeah, and enemies DO NOT respawn.  So no farming.  Souls are spent to level up, so each level is even more difficult to reach.
• Oh and I doubled the experience/soul cost for level 5 and upwards.  Level 5 costs 13,000 souls,  so it is a real crisis early on to risk saving or buying helpful stuff.
• Estus flask- lots of rules for this, since there are bascially no healing items.. starts at 5 charges 5 or 1d4+1, depending on if you use an action or bonus action to drink it (and you can drink twice in a turn if you want). I've considered if characters are able to share estsus but currently I've decided against it.
• Right, and initative is set for all enemies at a single DC (normally 10).  Players can take there turns in whatever order they like- winning initative effectively means having an extra turn.  You would not believe the literal hours I've saved doing this.

Using these changes, the Catacombs (which the players wanted to go to cause it looked fun lol) of all places have been a stellar time.  Every enemy is placed effectively and nothing feels easy.  Removing death saves is the most effective contributer of increasing tension.  The obvious enclosed dungeons- Catacombs, The Depths, Sen's Fortress- were expected to work well given their pretty clear progression structure, though some of the more open areas- Darkroot Garden, Undead Burg- might be a problem given the existence of certain knee high fences which lead to nothing and/or death and need to have new paths created;  limiting the usage of flight and climbing was something I tried to avoid given that's the whole point of choosing those options.  Here are some flexible non-gameplay considerations I've made:

• Do not make the excuse of having Lordran be "Video Game World" unelss you want a very unimmersive experience (sometimes silly is better).  Figure out where a character goes if that's the way they want to go but make the path forward clear.
• Reward players for breaking outside the normal limitiations of the game.  A point of inspiration, for sure.  So many collapsed stairs in this game...you'll be giving these out a lot in open areas if your players make an attempt.
• Planning for new endings.  Yeah, Dark Souls is dark, but give the players some hope that a new ending is possible depending on their choices.
• Consider non-combat strategy.  Since resting a bonfire is a resource, avoiding combat altogether can be a wise plan.  Yeah, most enemies have gone insane and won't understand reason, but that doesn't mean intmidation or gestures of peace are impossible.
• No repeating most abilitiy checks until resting at a bonfire, for the entire party.  No going through every character to bash a stuck door or finding a hidden wall.  Combat and risky checks make this rule flexible.
• Give NPC's new motivations and be know what their knowledge and intentions are.  Don't make them static actors and quest givers.  Some NPC's already got this covered cause they pretty much immediately betray the party, but allow characters to discern their intentions and change them.
• I've been lucky enough to not have anyone extremely familiar with the games to be a player, but if you do, this a good lesson in dealing with metagaming.

Players have been breaking down doors and going over "knee-high fences" a lot, getting items early, and on one occasion using humanity to gain a follower.  Unfortunately, these moments can be spread out far apart given combat still can be quite lengthy.  Getting through the catacombs has taken a couple of months with weekly sessions, though this is mostly aleviated since every combat contributes to something and isn't just a roadblock.

tl;dr:  Dark Souls 1 has great dungeon design and you can maintain the feel in D&D with a handful of somewhat non-intrusive changes.  Most work is done to make the world feel more real and reward player imagination.

I'm interested to hear what you think, perhaps there is a change I might need to implement or a problem I haven't foreseen?  I've seen (and drawn from) other conversions so I'm curious if anyone else has tried it before and how it went.  Also, I am most likely pausing this campaign soon to work on another one- funnily enough, I don't actually enjoy running the constant combat all that much!  I like getting invested in worlds, but if you just want a campaign where you don't really need to take notes (unless you want to avoid getting lost).  I would like to try this again with a group that was more familiar with the games to see how they play.  I also wonder if anyone has done any "secret" campaigns when making characters, since that is usually something I would avoid like a poison swamp without a rusted iron ring.