The mod is available on the Steam Workshop as a single click-and-play file due to its enormous size (over 20GB). Instead, it is hosted on ModDB and other community sources. Below is the safe, official method.
: Historically accurate unit rosters spanning the 13th to 15th centuries. Steam Community for your first 1212 AD campaign? total war attila mod medieval kingdoms 1212 ad campaign link
Some planned features like full naval battles (Attila’s engine limitations) or certain emergent factions were never fully implemented, but the experience is 95% complete and more polished than many official DLCs. The mod is available on the Steam Workshop