Hello everyone, this is my first post here so excuse me if I am writing this in the wrong topic.

*MINI SPOILER INC*
I have recently bought Skyrim Legendary Edition and I made a new character. As I played on my brothers Steam account with Skyrim on it, my character didn't save, so I had to make a new one. (obviously xD) So I made a new character and I just wanted to try out the DLCs as fast as possible, because my brother didn't have the DLCs. ANYHOW, I am doing the Dawnguard-pack and I have joined up with the Vampires, the mission is to get 3 elderscrolls. I have gotten 2 so far, and I am trying to get the 3rd Elder Scroll, which is:
Elder Scroll (Dragon)

BUT, here is the problem: The Elder Scroll is part of the main quest, and I hadn't really done the mainquests, but I went to do it anyways cause I really wanna go further with the vampires, I meet Septimus Signus cause that is what you have to do to get the scroll, and he tells me to go to some cave called Alfthand. I go there, but when I am going to enter a room to get further in the cave, my Skyrim closes. I restarted the game, changed graphics, deleted some saves, etc etc.. I realize I will have to cheat to get further, I do it and return to Septimus Signus. But I later realize I forgot to ask him about the scrolls and that stuff. And later on a quest, Septimus dies. And yea, that's what happened. I return to the Orc in the Winterhold Mages guild but he doesn't tell me anything. I checked on Skyrimwikia and everything, and to be able to get the scroll, I have to return to the Alfthand cave... But as I wrote, I can't because my Skyrim automatically closes. What am I supposed to do? I tried to use (player.additem *and the scroll code) but it doesn't work, I don't get anything in my inventory.. maybe I am using the wrong code... What can I do to get the Elder Scroll (Dragon)? Please, anyone.. Help me!

(Sorry for Wall of Text)
/GymShark

You can try entering Blackreach through one of the other dungeons that accesses it.

You will need that Dwemer sphere thing you got from Septimus to open the way.

You can find the other two Dwemer ruins that give access http://www.uesp.net/wiki/Skyrim:Blackreach.

Once in Blackreach find the Tower of Mzark. The scroll will be there. You can exit to Skyrim worldspace via the Tower of Mzark, but not enter from there until you unlock the door from the inside.

Good luck

You don't need the main quest for access to Blackreach. In the future you can just go straight to Septimus.

YES! THANK YOU MERARI!!! Oh my god, I wanna make love to you, thanks a lot mate! I thought I was done with :[, that I had to make a new character. But then you come fix it all like a guardian angel.. Thank you man/girl, I don't know how to repay you! <3

/GymShark

You're welcome

Edit: Actually, that edit came out a little nasty. Just ignore that.

you can cheat your way throught 1st transform to a vamp lord there is something like a tower near the entrance go there next to the corner there is a lever jump and try to open it very easy (remember you have to be a vampire lord or you wont reach it enter from there inside there is a master lochk open it and here you are.

PS:you can also try noclip at the console command to pass throught the door