LostArk is an upcoming PC game from Korean studio Smilegate. It looks nutso.
Once you get past the visuals in this trailer - and yes, most of this is showing what looks like gameplay - know that this isn't a straight action/RPG like Diablo. It says it's actually fusing that style of action gameplay with MMO elements, but from the trailer that really just looks like it has 4-player co-op.
It's been in development for three years, and The Korea Times says there'll be a closed beta next year. It might come to the West, it might not!