Here are the main rules:
* Life Pod Flotation Failure: your Life Pod will sink to the bottom, and will hopefully land at an extremely awkward angle! You will have to adapt to it! It will "right itself" to a more normal angle when you repair its secondary systems (but it will stay sunk). [Note - it will still hover a bit over the "ground" because I didn't want players clipping through the bottom of the world, and I'm not exactly an ace Unity programmer, just a friendly videogame designer who got left alone with the compiler]
* Surface Air is Unbreathable: you can't breathe the surface air, and can now asphyxiate on the surface! A floating pump, however, will sufficiently filter the air that you can breathe from the pipes.
* Increased Damage: all damage sources will deal you substantially more damage than you have been used to. No more "love taps" from creatures and surviving a whole bunch of Reaper Bites.
* Expensive Power Costs: power costs (especially for fabrication) have gone up substantially, and are further increased if you are in a radiated area. Exiting from the Seamoth and/or Prawn ALSO costs energy if done "at depth", except at Moon Bay and Cyclops. Power also charges/recharges much more slowly than it did before. The early game copper-based batteries hold less power and are not rechargeable - rechargeable lithium batteries come later.
* Radiation: Speaking of radiation, there's a lot more of it, a la the Radiation Challenge mod. The surface and the entire upper level of the ocean will be filled with radiation once the Aurora explodes, and the Aurora's explosion is much more powerful (go deep!)
* Aggressive Creatures: The ocean creatures will become increasingly aggressive. Don't be hovering in one place for long... The aggression level of creatures will gradually ramp up over the first 40 minutes of play. The distance they can "see you" is extended (but they still need a line-of-sight to you) and their likelihood of choosing the player over other targets is increased.
* Nitrogen and "The Bends": This mod includes a fresh re-do of the Nitrogen rules, inspired by the Nitrogen Mod but using a new formula. Among other things your rate of ascent will be important -- if you come up too rapidly you will have significant problems, even if you're below your "safe depth". When approaching the surface your "safe depth" will naturally settle out at around 3/4 of your current depth, so you'll need to get close to your limit without going above it, and then wait for a bit while it settles out. Pay attention to the tooltip entries, because certain common items and native life can be helpful in dealing with this new challenge. I'm a scuba diver in real life, and although I didn't want to turn this into a "diving simulator", I did reach for a bit more of the "feel of actually scuba diving" in terms of the safety of ascent. Watch out or I'll be adding nitrogen narcosis next...
* Personal Crush Depth: you have a personal "crush depth", initially of 200m. Better suits will be needed to reach deeper depths. There are upgraded versions of the stillsuit and reinforced dive suit allowing you to extend your crush-depth limits further. Blueprints for them are unlocked by scanning certain "denizens of the deep". You may wish to obtain a sample of said denizens with your trusty knife.
* Expensive Vehicles (and Habitat Builder): Key items (especially vehicles and the habitat builder) have difficult-to-obtain additional resource costs. Note that I have specifically avoided just "making everything cost lots more resources" because I didn't want to make the game "grindy" I wanted to make it "hard". Feel free to add on more other mods to make things more expensive if you like that sort of thing. [Note: some found my initial choice of Seamoth ingredients "immersion breaking", which is fair -- so I have provided "alternative DeathRun costs"]
* Stingier Respawns: although a true roguelike should of course be played "permadeath", Death Run does still support respawning "if that's your thing". But it's a stingier respawn, without as much free health/food/water as you may have been accustomed to in the past. Come on, you just got a free life, stop complaining and go eat some fish.
* Death Timer: Death Run will keep track of how long you live, and what kills you. Because you'll want to remember, of course.
* Lots of Feedback: Death Run provides quite a lot of additional feedback (in text form of course), to make it easier to learn its rules and what's going on. If (when?) you get tired of Nitrogen/Bends/Breathability/etc warnings you can turn them down or off in the Options.
* Challenging Start Locations: I have hand-picked some especially interesting and challenging start locations. I have tried them all and confirmed that they are "doable", but many may prove fatal the first time you try them!
* Top 10 Best Runs: Death Run keeps track of your "ten best runs"! Your scoring depends on several factors including how long you last, how many "Death Run" difficulty challenges you undertake, how many times you die (if you do), whether you build major vehicles (or complete the game during a no-vehicle challenge), whether you cure the disease, whether you escape the planet, etc. Note that if you die then "surviving for longer" makes for a higher score, but if you win then you get more points the faster you escaped.
Configuration and Options:
If you just don't enjoy one of the challenges (e.g. you like gasping for breath on the surface, but getting "The Bends" just drives you crazy) then you can individually turn anything off in the Options for the mod by setting it to "Normal" which restores vanilla Subnautica in that area.
* Difficulty Level: If you want to play this mod "the way I designed it", set everything to "Death Run" difficulty. Each setting can also be set to "merely Hardcore" which will still be more challenging, or can be set back to "Normal" which will return vanilla Subnautica function and challenge level to that specific area. For a few settings some individuals of doubtful sanity have requested a "Worse than Death Run" setting and I have duly obliged. I realize that many of these mods have lots of levers and sliders that let you personally configure things to an infinite variety of levels, but I come from a game design background where I want to create a finite number of "well-tuned experiences" and avoid "option overload and paralysis". That's why I've presented the configuration options in the way I have. (Oh and also I've tried not to introduce any options that make the game "easier" as that's not the point -- this mod is all about making things HARD... but interesting!)
* Murky Water: I considered making "murkier water" a core feature, but held back because it does affect one of the great joys of playing Subnautica which is looking out at the amazing underwater vistas. Therefore I have supported it but made it an optional feature -- if you want it darker and murkier (or darkest and murkiest) then by all means, check the option -- it's a scary experience!
* Food Sources: the Radiation Challenge mod, from which I drew inspiration (and with-permission, code) included features that restricted the extent to which food from the island can be used based on the radiation level. Because I didn't find that central to the gameplay and challenge I have tried to create, I have not made that part of the core Death Run experience. However the option to turn it on DOES exist for those who are "purists" and don't want their Radiation experience inadvertently made "easier".
* Escape Pod: There is a checkbox if you don't like your pod being "tipped over" at the bottom (but note it now rights itself automatically anyway when you repair it). Also, if you want the vanilla (non-sinking Escape Pod), you can pick "Basic Game" from the Starting Location list, and you will receive a normal start in the Safe Shallows.
