The Tomb of Kings is where the gargoyles for centuries have buried their deceased kings. It is located in the Realm of the Gargoyles, just south of the Shrine of Singularity.
However, visiting it without the caretakers is not advised, as the tomb is secured against looters with numerous force fields, magically locked doors, and countless beasts, even including the dreaded silver serpents.
The Tomb of Kings is not essential for finishing Ultima VI.