Requirements

• MHWNoChunk (or any other method of extracting the game's files)
• A hex editor, usual recommendations are 010 Editor and HxD
• (optional, but recommended) Stracker's move-subspecies program, which can be found here
• Stracker's Loader, which is needed to load the custom subspecies
• Aradi147's Quest Editor, the most recent version of which can be found in the MHW Modding Discord

File Setup

Extract all files of the main species of the monster you want to make a subspecies of (emXXX/00). Do not use an already present subspecies for this, as they do not immediately contain all of the needed files for the Subspecies Loader. If you want to use another subspecies as a base, wait until a little bit later into the guide.

Rename all files within the folder using the original subspecies ID (00) to your preferred ID, as well as any paths that may lie within the files themselves. If you are using move-subspecies, this is done automatically for you, just select original ID and preferred ID.

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From there, make your edits to the monster data as you would normally.

If you would like to use a subspecies as a base, do all of the prior steps, then also copy the files from the subspecies folder and replace the originals with these versions. You still need to rename any paths that may be present for this, so renaming the base folder, then copying the subspecies into your preferred id using move-subspecies, then merging the folders keeping all files in the subspecies-base one will work.

Loading a custom subspecies

Open a quest that you will fight the custom subspecies in the quest editor, and go to the Monsters tab. Edit the "Sobj ID" value to a unique value for the stage you want to spawn them on (sobjs can be found within quest/enemy/boss) and save the quest.

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Then, make a copy of a sobj for that monster/stage combination (or make one, should you need to) and rename it to use the proper ID. Now open the sobj in the hex editor. Find the first large set of CD hex values, and replace the last 4 with the int32 value of your subspecies ID.

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Now load up the game, enter the quest, and your custom monster should be present.

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NOTE: If you are crashing on load, make sure you have all necessary files and that these files are the most up-to-date versions. Both monster data files and sobjs have received updates that will cause the game to hang if they are not the correct versions.