The Empty Chair was a seat in the Rihannsu (or Romulan) Senate which (until 2276) was always kept empty in memory of S'task, the leader of the exodus from Vulcan centuries before. On the seat lay the Sword of S'task, the last of five legendary weapons forged by the smiths of Vulcan's Forge over five thousand years before, and carried into exile by the Rihannsu. The Sword had been placed there by S'task himself, in memory of his estranged teacher Surak upon the exiles' learning of his death in a civil war on Vulcan. (TOS - Rihannsu novel: The Romulan Way)
In 2276, the sword was removed by (then renegade) Commander Ael t'Rllaillieu of the warbird ChR Bloodwing, who believed that the Romulan people had lost their way. This sparked the Romulan Civil War which ended with Ael's installation as Empress in late 2276. Upon the recall of the Tricameron at war's end, Ael replaced the Sword in the Empty Chair and refused to take it for her throne, instead ordering that another chair be brought into the Senate chamber for that purpose. (TOS - Rihannsu novels: The Romulan Way, The Empty Chair)