Go through the narrow corridor to go back to the central area of the Black Stone. Use the switch there, and then run towards the north edges shown on the above screen. Get to the corridor on the bottom level (or you can just jump down).
Kill the Legion Soldier you'll encounter there, and use the opportunity to examine a nearby chest. When you're done, locate the time portal illustrated above and activate it with Phasewalker. Go through the gate to go back to the present.
After jumping out of the portal, turn left, which is west. Stop at the "regular" portal shown on the screen and activate it with Phasewalker.
Now, for a change, go east. Stop at the edge, exactly where the screen shows, and activate the portal on the wall.
Go back to the first activated portal (the one at the end of the west corridor) and jump through it - this will take you to a shelf on which you're seen a Book of the Dead page earlier ([The Book of the Dead] side quest).
Go through the portal again. Once you're in the west corridor (on the bottom level), look around the area. Locate the interactive ledges and handholds and move your way up.
Your destination is the west balcony - there, take the passage leading to the closed door you've opened while exploring the fortress in the other time zone (the above screen). When you reach the end of the corridor, you'll find a GnoMAD Gnome, connected with the [GnoMAD's Gnomes] side quest.
Return to the balcony, jump down and now, for a change, use the interactive ledge and handholds located on the east wall (screenshot 1). You should reach a large gate leading to the north chamber of the Black Stone (screenshot 2).
Follow the only available path. When you enter the throne room, go up the stairs leading north. Find the time portal shown on the screen and use Phasewalker on it. A boss battle awaits you in the past.