What works
Sign-in, matchmaking, groups and invites, private matches, friends, the shop, XP and career progress. The DLC and extra maps are in the playlists too.
Assassin's Creed III · PC multiplayer
Ubisoft's servers for AC III multiplayer are gone. This is a fan-run replacement, so Wolfpack, Domination, Manhunt and the rest can be played again.
Sign-in, matchmaking, groups and invites, private matches, friends, the shop, XP and career progress. The DLC and extra maps are in the playlists too.
Your own copy of Assassin's Creed III for PC. The download adds a small loader beside the game and points it at this server. You create an account for this server the first time you start the game.
Matches run peer to peer between players, like they always did. If you have trouble joining games, open UDP port 3074 on your router or turn on UPnP.