Skyrim Falskaar Volkrund Keep – How to Open the Door

It is the Dark Brotherhood's sanctuary.

To get through that door you have to

kill the owner of the orphanage in Riften and then a courier brings you a note. Sleep to be contacted by them, then they will give you the necessary information to join from there.

They may not contact you the first time you sleep so keep trying, eventually they will.

There are three runed tablets in the room - you can see one of them which has fallen by the lever at the right of your screenshot, the snake. The other two are above the portcullis.

Duplicate the pattern on those tablets by rotating the pillars, and pulling the level will cause the gate to open (instead of rain painful death on you).