Earlier today, concept work for something called Assassin's Creed Duel was listed as part of a portfolio belonging to former artist and UI designer for Ubisoft. It has now been revealed to have been fanart and nothing more.
Update: It's creator Benoit Perreault has said it was nothing but fanart, via comment posted here on the article's thread. This has also been noted elsewhere - and in the original thread from NeoGAF. So, in other words, move along. Nothing more to see here.
The working title was listed for PSN and XBLA, and while the images hold the disclaimer that the images posted are "not a game", the shots are located in a "Projects" section in his industry portfolio. He worked at Ubisoft for seven years.
The game appears to have been a 2.5D fighter in line with PlayStation All-Stars, and Smash Bros. based on the Ubisoft images, and at present we're assuming the game was in concept form only and either axed, put on hold, was only a proof of concept, or just from a brainstorming session.
Could be anything, really.
You can see the images for yourself below.