Tristan Seat
From Acw
The only Arthurian Seat to have dissolved, Tristan Seat stopped existing before the Splitting of Merlin.
History
The 'Lost' Seat
Tristan Seat was dissolved before the Splitting of Merlin when it was decided to merge it with Galahad Seat, as the two Seats had very similar goals. Tristan Seat was nominally to prepare for the terraforming of Avalon during the Return, but since Galahad Seat's jurisdiction of life support and agricultural maintenance had significant theoretical overlap, it was deemed in the Arthur's interest to combine the two houses and increase the number of people who had access to such vital technical information.
The Lost Server
The three unaccounted-for DPUs from the Merlin Server are explained by some rumours as the result of a never-completed Tristan Server. This is unconfirmed as yet.