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 death of the Third Doctor. He realised who the Seventh Doctor was quickly after only a few moments, and asked the Doctor how he could be in front of him when it was known he was dead.
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)