Brigadier Alistair Gordon Lethbridge-Stewart was the version of the Brigadier who lived on the alternate Earth ruled by the Silurians. He had been fighting them since they emerged from Wenley Moor twenty years previously. After the Seventh Doctor averted his plan to destroy the Silurians with nuclear missiles, he agreed to begin negotiations with them.
Lethbridge-Stewart was killed along with all the other inhabitants of that universe when the Doctor time rammed his TARDIS with the one he acquired there, destroying that universe. This was because the energy used by the alternate timeline shortened the existence of the main universe by billions of years. (PROSE: Blood Heat)