There are two distinct halves of the Fifth Tier. The first is reached via an elevator in a room in the Fourth Tier. This side of the area also includes the platform for the main elevator, but it won't be able to reach this tier when you first arrive. The other half of the Fifth Tier is accessible via stairs on the Sixth Tier. Here you'll find the mark for Mission 25 and the Menhirrim for Mission 26, as well as a fearsome Tyrant. There is another Menhirrim on the inner pathway, but you won't be able to access its power until you've completed the available Missions. Doing so will rotate the tier, putting you on the side with the Fourth Tier stairs, but without access to the elevator. Return to the Fourth Tier to reach the main elevator, which will take you to the final area of the Sixth Tier.