House of the Stewards
From Tolkien Gateway
The House of the Stewards was a building in the Hallows of Minas Tirith, situated near the House of the Kings on Rath Dínen.
It was capped by a great dome, and within was a wide vaulted chamber with shrouded walls. This was filled with marble tables upon which were carved forms of sleeping men - the tombs of the Stewards.[1][2]
It was the location where Denethor II immolated himself on 15 March, T.A. 3019[3] causing the building's dome to collapse.[2]