This trailer introduces Pixel Privateers, a collaboration between Terraria developer Re-Logic and Pixel Piracy developer Quadro Delta (thanks VG247). This Steam Community Group Announcement has early details, and here's a bit:

If that’s not enough to get you excited, this certainly will - Pixel Privateers is multiplayer, from day 1! We are extremely happy to make this an experience that can be shared with up to 3 friends, with each player controlling a squad. This time there is no waiting, no chance that it won’t happen, it’s already in game and fully functional. Keep your eyes peeled in the coming weeks for us to start showing the game on livestream and elsewhere, we believe you’ll love what you see!