Season 2, Episode 9:

In 2387, a major cataclysm of galactic proportion was averted when Ambassador Spock of Vulcan caused the Hobus supernova to be absorbed into an artificially created black hole. As a result of the supernova, however, the planet Romulus was destroyed and Nero sought to avenge the loss of his world. Both he and Spock were thrown back in time to the 23rd century. Nero then spent the next twenty-five years planning his revenge when Spock arrived in the past. Spock was then forced to bear witness to the destruction of the planet Vulcan in the year 2258. With ten thousand of his brethren left, Spock sought to preserve what was left his people while advising his past self in an alternate timeline to remain in Starfleet.

In 2375, while on a mission to destroy station Sentok Nor in orbit of the Dominion-occupied Betazed, Limis Vircona has a brief encounter with the Orb of Souls. The orb is accidentally opened during a scuffle with Cardassian and Jem’Hadar troops. The orb box is closed before it is able can induce any surreal visions. Things seem perfectly normal from her perspective upon returning to the USS Lambda Paz until she speaks the name of the Vulcan captain of a ship in the Seventh Fleet.

In this timeline, Lieutenant Commander Selek of the USS Epimetheus was never born as a result of Vulcan’s destruction a century earlier. In this timeline, the Federation is on the verge of defeat in the Dominion War. In this timeline, Starfleet is prepared to take extraordinary actions to assure victory over the Dominion, actions that compromise the Federation's own principles.