The reunion of the Vulcan and Romulan peoples on the planet Vulcan, renamed Ni'Var, which was achieved in the centuries after Ambassador Spock began the work and is revealed in Discovery.
The reunification of the Vulcans and Romulans is established in the Discovery episode Unification III. The Romulans descend from Vulcans who left their world long ago, and Spock was working secretly on Romulus for a reconciliation as early as the Next Generation story Unification. Arriving in 3189, Michael Burnham finds that her brother's hope was fulfilled long after his death. The survivors of the Romulan supernova were eventually welcomed home, and the planet took the name Ni'Var.
The union is uneasy. Vulcan purists, Romulan elders and people of mixed descent keep separate factions, and the Qowat Milat order is respected. Ni'Var left the Federation about a century before Burnham's arrival, because it believed its own SB-19 experiment had caused the Burn.
Burnham's appeal for the SB-19 data begins to mend relations, and Ni'Var rejoins the Federation in the fourth season.