Noticed this banner on the top right of the Steam store:
Weekend Deal
Play all Total War Titles now until 1pm pacific on Sunday.
Also save 33% to 75% off until Monday at 10am Pacific!
I also found this announcement post on the Total War: ATTILA page to confirm.
TLDR: All Total War games free to try on Steam from Sat. 3AM EST to Mon. 6AM EST