LOD archives are used by HoMM III to store most of its data files conviniently in one place. A single archive can store up to a hard limit of 10000 files, either compressed using zlib or included as-is, with a size limit for a single file of 2^32 bytes and identical offset limit - thus making it theoretically possible to cram almost 8GB of data into one archive, however pointless would that be.
The archive format itself is very simple, with a static header of 320092 bytes followed by raw compressed or stored data:
struct h3lod { uint32_t magic; // always 0x00444f4c, that is, a null-terminated "LOD" string uint32_t type; // 200 for base archives, 500 for expansion pack archives, probably completely arbitrary numbers uint32_t files_count; // obvious uint8_t unknown[80]; // 80 bytes of hell knows what - in most cases that's all zeros, but in H3sprite.lod there's a bunch of seemingly irrelevant numbers here, any information is welcome struct h3lod_file[10000]; // file list } struct h3lod_file { char name[16]; // null-terminated, sometimes null-padded too, sometimes padded with, well, something after the null uint32_t offset; // includes the header size, no preprocessing required uint32_t size_original; // before compression, that is uint32_t type; // what's in the file - probably not used by the game directly, more on that below uint32_t size_compressed; // how many bytes to read, starting from offset - can be zero for stored files, use size_original in such case }
The only complicated thing here is the type field, but it was probably only used by the NWC's development tools, the game seems to just ignore it. Most LOD-editing tools out there set it to zero when adding a file (proved by experiment, as not a single one of those comes with a source code). For the sake of completeness, that's the list:
There's probably some more I'll add as I notice them.