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As far as I know, even for offline play you need to run Uplay (seemingly owning a game on Steam isn't enough grounds for Ubisoft to assume you probably got it legally), so you will also need an account.
Steam can then be put in offline mode, too.
According to Ubisoft's FAQ:
As long as you have already logged in successfully once, the client is designed to switch automatically to offline mode if you do not have an active Internet connection or cannot connect to the game servers.
Also, if you wish to force the client to start directly in offline mode, you can tick the Always start Uplay in offline mode box in the client Settings, or via the main menu by selecting Go offline.
While in Offline mode, most features will be unavailable, but you can start your game from the Games tab as usual. Almost all games in our current catalogue can be played offline after they have been activated, providing they do not have any always online requirements, and your solo campaign will resume from the latest local save.
An example: you can play Assassin's Creed Origins completely offline for up to 30 days, after which you'll have to sign in to Uplay again.
Same goes for Odyssey.
I have just tested playing Assassin's Creed II through offline Steam through offline Uplay, and that worked fine.
Be sure to check for each game it's actually saving your progress while offline.