If those details are to be believed, Assassin's Creed Odyssey will indeed take place in Ancient Greece, with Assassin's Creed: Origins' Bayek and Aya filling a similar story role to the one occupied by Altair in Assassin's Creed II.
And if that dude getting kicked off cliff seems recognisable, it is because it is basically just that bit from 300.
The most recent entry in the franchise was Assassin's Creed Origins, released past year, which dialed the events of the game all the way back to Egypt during the Ptolemaic period (49-47 BC).
Beyond the tweet, information about the title's gameplay mechanics, storyline, and protagonist is scarce. We're expecting to see more at E3. The leak originally came from upcoming Assassin's Creed merchandise, which was spotted by French site, JeuxVideoLive.
The game is slated to release sometime during Ubisoft's 2019 fiscal year. Ubisoft is also considering how it can continue to grow the series after the successful adoption of more role-playing elements in Origins. Personally, the high points of the series for me remain Assassin's Creed III and Black Flag, the latter of which brought the delightful possibility of sea-traveling to the series. Does that mean the annualisation of the franchise is back on? If you recall, there wasn't an Assassin's Creed game released in 2016. That changed this year, with very little being said about the next game in the series.
As with all rumors, take this with the proverbial grain of salt.
Stay tuned for more as it develops.