The siege of the dragon cult quest is given by captain valmir at forelhost and in return you gain 1 word of power and rahmin's mask.

However, i killed valmir before entering the tomb and i didnt get the quest. And now, here i face the dragon priest, and he doesnt give the mask. Also the dungeon seems broken and I FREAKING GOT LOST INSIDE since i dont have the quest marker. I m up to the stage where a dragon priest spawns with 2 draugr deathlord, and above me there is a petty soul gem that does ice damage. Please help me to find my way out towards ranmir!!!!

It's a bug associated with killing Captain Valmir before he gives you the quest - a glass claw that you need to enter Rahgot's chamber will not appear, so you can't get the mask or get to the exit that leads to the word wall. Your best bet at this point is to reload a save from prior to killing Valmir and then let him give you the quest.

Dam, problem is i killed valmir before i finished the main quest, and that was 14 days ago.....
I M STUCK T.T
guess ima coc whiterun and use the console to get the mask

Thats what you get for jumping the gun killing doesn't solve everything and i got all the masks.