Turbine has posted an overview of The Twenty-first Hall on the Seventh Level of the mines of Moria.
The Twenty-first Hall of Khazad-dûm lies on the Seventh Level of Moria, leading away in many directions including toward the Second Hall and the Bridge of Khazad-dûm. Off to one side, a passage leads to the Chamber of Mazarbul, the dwarves'' hall of records, though it passes now for the tomb of Balin, son of Fundin.
The Twenty-first Hall was once a mustering hall, but has lain empty since the fall of Moria - though recent stirrings may change that. Recently, the Balrog of Moria came to the Twenty-first Hall seeking the first true challenge to his power since the deaths of Durin and his son Náin.
Fans of J.R.R. Tolkien''s The Lord of the Rings will recall that the Company of the Ring camped in the Twenty-first Hall as they passed through the Dwarrowdelf, before moving on to investigate the Chamber of Mazarbul.