It's based in the blade and soul world and kind of mirrors the game for parts of the story, since the main character's master was killed and they are looking for revenge. Some characters from the main storyline are also in the anime.
Just finished watching the anime to confirm that, yes, it is a spin-off. Same universe but different timeline, so all the characters from the MMORPG have different names and personalities in the anime. My guess is that the anime is designed to have an ending whereas the MMORPG doesn't. Also possible they went with different personalities and names in the anime for the MMORPG characters so they could try doing something different without messing with the continuity of the game.
Usually anime with a mostly female cast tend to be very fanservice-y, but the Blade & Soul anime luckily doesn't fall into that too much and actually has an interesting story and character development. There are some characters that I wish had more screen time, but the writers did a decent job with the limited amount of episodes. If you enjoy the story of the MMORPG, you might enjoy the anime as well.
Well, I'm not surprise if the anime and the game got different story. It will be not worthwhile to create another media which already exist. Same goes like .hack// series...they share the same universe but different timeline and all are linked together. That's their way to create more media for the franchise.