To unlock Raven Rock Mine, you must first talk to Crescius Caerellius, who will give you the key to the mine. Delving deeper into the mine, you will obtain the Bloodskal Blade. This is a unique Two-handed Greatsword that has the unique effect of launching arcs of red energy. You will need to launch the ranged attack horizontally at the specific locations surrounding the Bloodskal Barrow door. You'll know the door is unlocked when the door begins to glow red.