Assassin's Creed IV: Black Flag, aka the best Assassin's Creed game, will be free to download on Uplay next week. Not for a limited time, and not if you jump through a series of fiery hoops... it's free for you to keep forever, so long as you add it to your library between December 11-18.
Meanwhile, 2007 RTS World in Conflict: Complete Edition is free to download right now on Uplay, and will be until December 11. The game is set in an alternate 1989 during World War 3, and is developed by the studio responsible for The Division, Far Cry 3 and Assassin's Creed: Revelations, among others.
And while that's a very worthy (albeit old) RTS, you can't go past Black Flag at this price. Our reviewer Tom described it as "a gorgeous, relentlessly entertaining open-world piracy simulator packed with interesting 18th century rogues."