So I am about to do the thieves guild quest where you have to track down and kill Mercer. Is it possible to bring in a follower with you into the battle. I know that sometimes the game makes your follower stay behind.

You can bring a follower but that follower will be left outside.

I have played the mission before but on this character I'm trying not to do combat. I grabbed the follower from the new DLC, but you say I will still be alone?