NIGHTMARE V1.6.1  

I HAVE NOW ADDED A SEPERATE VERSION THAT REMOVES MOVING AROUND OF KEY ITEMS, MAKING IT COMAPTIBLE WITH THE RANDOMIZER.

BIG UPDATE!
Nightmare is a mod that greatly increases the game's difficulty by changing several factors. Things like increased merchant prices, changed items in the shop or modified damage values, key item locations, reworked AI will surely require a different approach to many situations. Any difficulty can be used as preferred. There is also an Inferno Version (Now Updated to V1.4) for even faster enemies!

NOTE: Make sure your download folder of the MOD has "V1.4.1" to make sure it is 100% Latest Version. V1.4.1 fixes a bug that could occur on chapter 13 NG+, and changes the location of the Biosensor scope for all versions. 

Be sure to Re-read game archives before installing the mod, it is important to make sure that all the changes are present!

Difficulty Recommendation

I recommend Standard Difficulty on the original version NIGHTMARE for a first run. Hardcore and Professional can be quite brutal in some areas, especially on the Inferno Version. Not only the damage changes between the difficulties, but also the saving between Hardcore and Pro. 
The mod is meant for a New Game on any difficulty, with no Bonus Items/Weapons or Accessories, but it is up to YOU to choose how to play.
I have cleared all Versions of this Mod on NG Inferno Professional, so it is possible.

DAMAGE
Enemy Damage is greatly increased in this mod according to the difficulty that is chosen. Most attacks are survivable in All Difficulties, however, there will be some attacks that are not, or barely survivable with the right conditions. Hardcore and Professional can be quite unforgiving with damage and attacks that enemies do. It is meant to be HARDER than the regular game. This makes YELLOW HERBS and ARMOR extremely important for survivability, especially since it will be life or death without these on Hardcore or Professional to some attacks. Doesn't hurt to try Standard first!

INVENTORY
The Starting Inventory has now also been changed, and you begin with new gear. The SG-09 R is still available from the Merchant... for a price. You also begin with an extra 5000 PTAS, and 10 SPINELS.

MERCHANT
The Merchant has now been changed, and offers new items, different items, as well as double price of all items. This includes repair costs, which are more than double. Selling also will NOT be as beneficial, as the price for selling items are less than before. Take care of your gear, you never know when you may need it most!

SPINEL SHOP
Along with the Merchant, the Spinel Shop also has received changes to make Spinels more useful. Some items that were originally here are either gone, or have received a lower/higher cost. There are also a new variety of items for purchase such as Armor (you do not repair it with Spinels), Killer7, Attaché Cases and more! The prices may be a bit high, but you are also rewarded with additional Spinels in this playthrough for your efforts. The YELLOW HERBS are INFINITE!

Merchant Requests and Medallions
Merchant Requests also have received some changes. Medallions are no longer in the same positions they used to be, and will require careful looking in order to spot these things. They are much harder to spot than before, and are no longer visible in the map (the correct location). If you managed to find them, you will be rewarded with THREE more Spinels than normal. This also includes changes to the STRONG FOE requests, which will also grant you FIVE additional Spinels, in exchange for a tougher fight. GOOD LUCK spotting these things, they may not be easy.

ITEMS
Most of the Key Items, as well as some Jewels and ALL Small Keys are no longer in their original positions. Look carefully for where they may be. The treasure map may still prove useful if you are NOT aware of all the original treasure locations, as a ITEM may be there instead. 

Crafting
Crafting also received many changes for what can be crafted, the resources required, and for what you can make. Most new recipes are seen in the Crafting Menu.

NOTES:
Some items in the spinel shop may have a wrong icon or description. The item you see there by name, is what it is.  The MATILDA is a good source of extra money!
The Mini-Map does NOT show the correct location of keys and Medallions. It is for YOU to find.
Starting from a save file in the middle of the game where it was NOT played on V1.4 may cause a softlock, such as Chapter 8 before putting in the Lithographic Pieces due to the new locations.
For players playing on NG+, the first regenerador will not spawn on Chapter 13 as I have heard due to no bioscope. The best solution to this currently is to uninstall the mod and grab the keycard, then toggle the mod back on.

I HAVE NOW ADDED A SEPERATE VERSION THAT REMOVES MOVING AROUND OF KEY ITEMS, MAKING IT COMAPTIBLE WITH THE RANDOMIZER. IT IS UNDER MISCELLANEOUS.
If you do not have knives for Krauser Part 1 (Chapter 11), I recommend using the Primal Knife in order to progress that section to avoid a softlock.

Download:

INFERNO V1.6.1
NIGHTMARE V1.6.1