That creepy alien is one of the members of the Black Order: Ebony Maw. He is a master of manipulation and sorcery and has the ability to affect people's minds with "webs" that give him information about that person's secrets. He is the mastermind behind the Black Order, and I think Thanos will send Maw at Strange to try to retrieve the Time Stone, since he seems he to have a similar skill set to at least challenge Strange. In the scene it appears Maw is getting the better of him and using some of those webs/shards to torture Strange and gives Strange a very malicious “Shssh”. This scene once again gives weight to the idea that the Avengers are going up against a threat they have never really seen so far in the MCU. Hopefully this helps!
Are you interested in cosplay? If yes, please go to here with the avengers cosplay costumes