The accident in 2266 in which Fleet Captain Christopher Pike was crippled by delta radiation while rescuing cadets from a ruptured baffle plate aboard an old training ship.
Pike had left the Enterprise and was inspecting a class J starship used for cadet training when a baffle plate gave way. He went in repeatedly to drag out the trainees he could reach. The delta rays left him paralysed, scarred and able to communicate only through a light on his chair.
The accident is described by Commodore Mendez in the Original Series episode The Menagerie, in which Spock takes his former captain to Talos IV so that he can live free of his body.
In the Discovery episode Through the Valley of Shadows Pike sees this future in a time crystal on Boreth and accepts it. Strange New Worlds follows him as he lives with that knowledge, learning the names of the cadets he will save.