The destruction of Vulcan by Nero in the Kelvin timeline.
In the alternate timeline created by Nero's attack on the Kelvin, the Romulan miner returned in 2258 with a mission of revenge. He blamed the Vulcan Spock for failing to save Romulus from a supernova, and decided to destroy Vulcan as a way of making Spock suffer.
Nero used red matter, a substance capable of creating black holes, to drill into Vulcan's core. The Federation fleet was drawn into a trap and the planet was destroyed, with only a small number of Vulcans escaping. Captain Pike was taken prisoner, and the young cadet Kirk was left to defend the Enterprise with Spock, who had lost his mother Amanda.
The destruction of Vulcan was a defining moment for the new timeline. The surviving Vulcans, led by Sarek, started to build a new society, and Spock committed himself to helping them. Kirk and Spock found a way to defeat Nero, and the tragedy marked the start of their partnership. It changed Starfleet's view of its enemies and its responsibilities, and it cast a shadow over the early missions of the Enterprise crew.