Here's the scoop as reported at Eurogamer:
(...) Total War: Arena. It's a free-to-play online MOBA-style spin-off.
In it you "pitch history's greatest commanders and their armies against each other in massive team-based battles", announced the announcement.
Arena is entirely online and mixes RTS and MOBA gameplay. It's in development at Creative Assembly's UK HQ, is presumably a PC game (...)
This confirms some prior intel that had the name of the game datamined last month in connection with some anonymous Steam apps.
The game will be adhering to a free-to-play model and, again, according to Eurogamer, it seems signing up for the closed beta will be possible through this link, though at the moment it looks to lead to a dead end.