While the base Aurora Toolset can allow a module to be created (with areas, creatures, conversations, items, stores and scripts) you might want to use custom content in your module or create custom content.
This can include new models and animations (for creatures, placeables, items, VFX and tilesets), textures (for models, but also including portraits, icons and special colourable PLT textures), game data (spells, races, classes, feats, item types and much more), sounds, music, shaders, TLK files for text, movies and several miscellaneous things.
Getting Started with Custom Content
For an introduction on how to add a simple placeable to the game files, see: Introduction to Adding Custom Content to a Module. For hakpacks to bundle your custom content for release see Introduction to Hakpacks.
For more tutorials see Tutorials covering a wide range of topics.
Custom Content Tools
See the Custom Content Tools page for a more exhaustive list, here is a basic rundown for essentials;
For exporting game files you'll use NWN Explorer or the command line nim tools.
For modelling, you will want Blender and NeverBlender plugin, with potentially Clean Models: EE for decompiling existing exported models as examples.
For texture work you need NWN Crunch to convert from source files like TGA or PNG to better suited DDS. For PLT editing there are a number of tools available, including a GIMP plugin.