Marcus was employed by Spike to torture Angel into revealing the location of the Gem of Amara. When Spike got the Gem, it was stolen by Marcus, making him invincible. Angel tracked him down though, and pushed him onto a plank of wood before removing the Gem, and killing him.
Before becoming a Vampire, Marcus was probably an executioner in Europe. He is probably about 200 years old or more. He loves torture, and it is suspected he invented some of the classic torture methods. I can't say much about his fighting abilities because he didn't fight very much, and when he did he had the Gem's powers on his side.