And before you ask, no, there are no paid mods.
The Creative Assembly and Sega have released the official mod kit for their latest grand strategy game that released back in February, Total War: Attila. It’s called the Attila Assembly Kit, and though it is still currently in beta (technically), it is already the most extensive set of modding tools in franchise history yet.
The Attila Assembly Kit offers four separate editing tools- DaVE (database editing), BOB (processing of raw data), TeD (creation of individual battlefields for land, sea and sieges), and Terry (campaign map editing). It also offers a Terrain Raw Data resource pack that includes a a staggering 60 GB of in-game production data to play around with, should you want to.
Also, The Creative Assembly and Sega have both confirmed that this game will not be supporting paid mods. Unfortunately, it seems like this is a question most companies releasing modkits for their games on PC via the Steam Workshop will have to answer for a while now.