Skyrim mods are a varied and numerous bunch. Some add new weapons and spells; others fix and tweak systems; the vast majority give Lydia new hairstyles. Occasionally, there's another type: an unbelievably detailed and complex quest chain, complete with new locations, quests and dialogue. The Forgotten City is from this latter category, and it's shaping up to be a fully-fledged expansion to Bethesda's four-year-old RPG.

"The Forgotten City is a lore-friendly expansion to Skyrim with a dark, non-linear story in which you'll discover and explore an ancient underground city," explains its creator. It's designed as a murder mystery, in which players must search for clues, interrogate suspects and fight monsters. And also travel through time.

The mod will boast 1,200 lines of dialogue, 30 new characters and a moral choices that can lead to multiple different endings. It's an undeniably impressive undertaking.

For more on the mod, head over to its ModDB page. The Forgotten City is due out in October.