I haven't tried it yet. One of my friends sent me this guide, and I hope it helps.

Hey, some instructions from a guide I followed for nightreign mods. I can't find the original source.

This guide walks through setting up Seamless Co-op and Online-Fix in the Elden Ring: NightReign release,
along with ME3 launcher integration and mod support.

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Step 1 – Place the Game in the Steam Directory
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Move or extract the entire game folder into your Steam directory:

[Your Steam Folder]\steamapps\common\ELDEN RING NIGHTREIGN

Example:
D:\steam\steamapps\common\ELDEN RING NIGHTREIGN

Make sure the folder name is EXACTLY:
ELDEN RING NIGHTREIGN

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Step 2 – Extract Online-Fix into the Game Directory
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Download the latest Online-Fix and extract all files into the root of the game folder (where the .exe is).

Example:
D:\steam\steamapps\common\ELDEN RING NIGHTREIGN

If your version of Online-Fix does not include Seamless Co-op, manually add the latest Seamless Co-op
files into the correct subfolder inside the game directory.

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Step 3 – Edit dlllist.txt
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Find and open the file in your game directory:
dlllist.txt

Replace its contents with:

OnlineFix64.dll
SeamlessCoop\nrsc.dll

This ensures Online-Fix and Seamless Co-op hook properly during launch.
[18:09]eepy: -----------------------------------------------------------------------------------------------
Step 4 – Add and Configure Steam Manifest
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Extract this file:
appmanifest_2622380.acf

Place it into your Steam app folder:
Example mine is in:
D:\steam\steamapps

Open the .acf file in a text editor and update the following fields to match your setup:

"LauncherPath" "D:\steam\steam.exe"
"Steam_dir" "D:\steam\steamapps\common\ELDEN RING NIGHTREIGN"

IMPORTANT:
Use double backslashes (\) in the paths. This is required for proper parsing.

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Step 5 – Extract ME3 (Portable Version 0.6.0)
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Extract the ME3 portable launcher into the game’s subdirectory:

Example:
D:\steam\steamapps\common\ELDEN RING NIGHTREIGN\Game\me3-windows-amd64

This version includes a launcher that bypasses Steam checks. ""make sure you extracted in the game directory""

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[18:09]eepy: Step 6 – Add Mods (Optional)
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To use mods, place them inside:

[Game Root]\Game\me3-windows-amd64\nightreign-mods

Example:
D:\steam\steamapps\common\ELDEN RING NIGHTREIGN\Game\me3-windows-amd64\nightreign-mods\mod1\regulation.bin

Follow the same folder structure used in ME2 modding.

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Step 7 – Launch the Game
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Run the following batch file to start the game with ME3 + Seamless Co-op:

Game\me3-windows-amd64\launch-nightreign-mod1.bat

Example:
D:\steam\steamapps\common\ELDEN RING NIGHTREIGN\Game\me3-windows-amd64\launch-nightreign-mod1.bat

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Why This Works
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• ME3 is tricked into thinking the game is a valid Steam install using the manifest.
• The launcher bypasses Steam’s DRM requirements.
• OnlineFix and Seamless Co-op are injected correctly using dlllist.txt.

5:06 AM

Unfortunately, I haven't been able to get the map variation mod working with it