The Spirit Altar

The Spirit Altar is a map feature located in its own gated chamber on the eastern side of the Watcher's Keep Final Seal dungeon level of Watcher's Keep.
There is an Automap notes marker on the area map showing its location.
It can be interacted with using the player's cursor to generate certain actions necessary to advance the Unlocking the Final Seal questline.
==Gameplay==
When the player opens the gates to the chamber where the altar is located, there is a group of hostile golems positioned in close proximity.
If the party member attempts to interact with the altar (using the cursor) while any of the golems are still alive/present, the altar generates an on-screen text, reading: 
:You don't think it would be wise to touch this device while in combat.
Once the golems are defeated, clicking on the altar displays the following text:
: This altar bears inscriptions referring to the skull of a noble warrior, but you can find no skull nearby. If the altar is to be of use, you shall need to find this Warrior's Skull.
After the party has obtained the Warrior's Skull (via the Helmite Ghost challenges), then clicking on the altar displays the following:
:This altar bears inscriptions referring to the skull of a noble warrior. You place the skull of the Eternal Warrior from your pack on the altar to activate its magic.
This action removes the skull from the party's inventory and executes a cut scene (Scut01.bcs).
The entire party is eventually moved to a new area, and none are selectable or controllable.
A new concept is introduced into the gameplay during this scenario, where Gorion's Ward is now rendered on the new area map and is titled as the Spirit Warrior in the dialogue box. 
Apparently, the main character is currently embodied with a new ghostly form, an "Eternal Warrior" of some kind.
For what transpires next, see this linked article page.