This mod represents a comprehensive custom outfit and body overhaul for the character Lune. The project is currently in active development.

This is a fan-made modification. It is not affiliated with or endorsed by the original game developers or publishers.

When modding Lune, you will encounter several different Unreal Engine file formats. Understanding their purpose helps you know which ones matter when creating or replacing assets.

The most common are .pak and .pac files. These act as large container archives, similar to .zip files, and hold the game’s packaged assets such as textures, meshes, animations, and audio. .pak is the standard Unreal Engine archive format, while .pac is a variation used by certain publishers but serves the same role. Mods usually work by creating or overriding these archives so that new content replaces the original.

More recent Unreal Engine versions (UE 4.25+ and UE5) introduced .ucas and .utoc files, which work together as part of the virtualized asset system. The .ucas file contains the actual bulk asset data, while the .utoc file acts as the index or table of contents, telling the engine where specific chunks of data are located. The game always loads the .utoc first and then streams from .ucas. Both are required for assets to function properly.

You may also encounter .usmap files. These are not assets themselves, but metadata that define how Unreal Engine objects are structured. They contain property names, type mappings, and serialization layouts used for .uasset and .uexp files. Many games strip this data to make reverse-engineering harder, which is why external .usmap files are often needed. Tools such as FModel or UAssetGUI rely on .usmap to correctly parse and display assets; without it, the data appears scrambled or unreadable.

Finally, .pskx files represent skeletal meshes. This is an extended version of Unreal’s older .psk format and is typically produced when extracting models with tools like UE Viewer (UModel). .pskx files store 3D mesh geometry, bone hierarchies, skin weights, multiple UV sets, and normals. They are essential for character and outfit models, and are often paired with .psa files, which contain animation data. For mods that involve dress changes, .pskx files are the core format you’ll be working with when swapping or editing models.

In short, .pak and .pac are the high-level archives, .ucas and .utoc are Unreal’s newer bulk data and indexing system, .usmap provides the metadata needed to read and edit assets, and .pskx holds the actual 3D skeletal meshes used for models such as outfits.

When creating a dress change or model replacement mod for Lune, not all of these file types require direct editing. Some are core game assets that the engine expects to remain intact, while others are the formats you actively work with.

The files you will directly modify are primarily the .pskx skeletal meshes, which contain the outfit or character models. These are extracted, edited, or replaced using UE Viewer (UModel), Blender, or other 3D software with proper import/export support. Once you’ve prepared your custom model, it can be repackaged into a new .pak (or .pac) file so the game loads your changes instead of the default assets.

The .pak or .pac archives are the delivery format for your mod. You don’t usually edit them by hand; instead, you repackage your modified assets into a correctly named archive so the engine recognizes and overrides the originals.

The .ucas and .utoc files generally remain untouched. They belong to the game’s official virtualized asset system and handle bulk data streaming. As a modder, you don’t recreate these—your replacement .pak files coexist alongside them.

The .usmap files are needed only for asset inspection and interpretation. They allow tools to correctly parse .uasset and .uexp structures. In most cases, you won’t edit .usmap yourself, but you will rely on it when extracting data or analyzing game assets with third-party tools.

Download and place the files in the following folder structure:

Expedition 33\Sandfall\Content\Paks\~mods\

If you do not have a ~mods folder, create one and place your mod in the correct structure.

There might be some unusual clipping when Lune is climbing ledges. This is a limitation of the mesh replacement system and may require further adjustments to weights or animations in future updates.

During the asset pipeline process, specifically when handling the base SK_UE5_Mannequin skeleton, a structural inconsistency was identified. The original mannequin skeleton includes both:

• A bone named root
• A socket named Root

When importing the corresponding .psk file into Blender, sockets are implicitly interpreted and converted as bones. This behavior results in the creation of two distinct bone entities: one named root and another named Root. Despite being semantically distinct in case-sensitive environments, this duality leads to namespace conflicts upon re-import into Unreal Engine.

Unreal Engine, upon encountering the name conflict during import, resolves it by automatically renaming one of the conflicting bone references. In this case, the socket-derived bone is renamed to Root1. While this mitigates the direct naming issue, it introduces a non-trivial deviation in the skeleton hierarchy and potentially impacts animation retargeting, bone weight mapping, and physics asset behavior.

A runtime issue has been observed, specifically a freeze or stutter during jump animations. While conclusive evidence is pending, it is hypothesized that this may be related to the presence of redundant or misnamed bones (root, Root, Root1). The conflict may cause anomalies in animation blueprint execution, bone-driven controllers, or physics simulations associated with the character's vertical motion states.