Stay in Lilith's dungeon in the [Black Stone] and look around for a Book of the Dead page ([The Book of the Dead] side quest). Go back to the first area of the fortress and go up the previously ignored north staircase to get to a new door (the above screen).
Without a sweat, eliminate Undead Scarabs in the central area of the Black Stone. Afterwards, select Phasewalker from the ability list and locate the time portal illustrated above to travel to the past (the middle mouse button).
In the past, start by exploring the first balcony and collecting a Boatman Coin. Then, move to the nearby arena where you'll have to fight a Legion Champion. The opponent uses mostly a chain which can not only deal damage, but pull your character to himself (the above screen). It's crucial to dodge effectively so as not to let the Champion take over the control of the battle.
While fighting the Legion Champion, I surprisingly don't recommend using Death Grip, but instead just the standard melee attacks. Make use of the fact that the boss is fairly slow and can't respond to your offensive at once. Consider using special attacks (such as summoned helpers shown on the above screen). The only exception is the Reaper Form - it's best to save it for more difficult encounters.
Further in the battle, Legion Soldiers will also appear on the arena. Since those demons are visibly weaker and don't have chains to use, take them out first. You can pull the Soldiers to you with Death Grip, they're quite vulnerable to it. Nevertheless, don't think that eliminating the first batch of them will resolve the issue, because other beasts will soon join in as well. Same as before, focus on attacking the Soldiers, leaving the Champions for last.
When the fighting is over, look around the area for loot dropped by enemies; you'll also notice that the only available passageway is a door in the south-eastern part of the chamber (the above screen).