Run straight ahead. Go pass the ships and reach the ladder leading upwards. Reach the closed gates and open them.
Check the Lorestone and run up the stairs. After reaching the gate - open it. Darkness will appear.
Run as fast as possible towards the light to avoid the darkness. Then run towards the torch and snatch it - this will provide temporary shelter from the Darkness. Take three runes from the door, turn around and run down the stairs.
Run towards next lights, and when you'll get a torch, light up the bonfires.
You'll reach a flooded hallway. Diving is mandatory, so the torch will be extinguished. Go forward towards the light. Then run down the hallway and stop at the next light. Repeat that a couple times, until you reach a fireplace with a torch. Pick it up and light fires around you.
In the flooded cave you will find a Lorestone. When you light up all the fire places, you are able to create the "P" rune.
The enemies don't stop pestering you even in the dark.
Run up the stairs. On the left, in a nook, there's another rune. Unlock the door and go further. Go back to the entrance door and turn right. Take the dark hallway and then the stairs down. After a while, enemies will appear. Defeat them to continue.
After the fight, light up the torches and go up the stairs. Look on the blood beneath you and you'll get another rune. Go back to the main hall with rune door and enter the entrance behind the stairs going up.
Go through the dark corridors and light up the torches. Then look for the last rune on the wall and go right. Open the passage and approach the rune gate to open it. A beast will start chasing you.