A vampire lurks in this room as you enter. Defeat it then speak with the Menhirrim for Mission 25. Choose to save if you wish then head out through the doorway where a single Varcolaci patrols. Kill it and move into the next area and a trio of Chonchons will attack you. These Cie’th are like giant bats and are fairly weak but can inflict Pain and Poison. The only other enemies are Varcolaci.
Talk to the Menhirrim to have it open up new areas in the Tower and then go back out to the circle. Ignore the room you’ve already been in and instead enter the second room on the right which contains another Vampire and the mark (which is just downstairs) you’ve been looking for. First, open the Sphere for 4x Tears of Woe and another in the opposite corner for the Unsetting Sun (weapon for Snow which increases Strength but has Stagger lock), then descend the stairs to return to the Fifth Tier.